General
At mid-high levels at-will powers aren't going to be especially useful at damaging a foe, and the amount of control they supply is rather limited. Thus, most of the heavy lifting will need to be done by encounter powers and daily powers. However, 4 encounter powers is probably not enough to see you through combat, meaning you will need to dip into your supply of dailies, and severely limiting your endurance as you may well tap out after 4 encounters. If you somehow had access, however, to more offensive encounter powers, that endurance could be greatly increased.
Wands provide a metric for increasing your encounter power usage at the cost of burning a daily resource. However, at low-mid levels this will require a competitive wand to be worthwhile, and thus demand the wand as your primary implement (and with only a single wand, the increase in available daily powers is no more significant than the abilities of other implements, and likely less). Two powers cause the carrying of backup wands to become worthwhile - Mesmeric Hold and Force Volley - because they supply an additional +4 to hit.
A wizard need not be a wand specialist to benefit from this - her primary implement can continue to be her specialist tool, and she can pull a wand to use its power as needed. As such, this has general applicability to all wizards.
As the item level for the lowest of these powers is 18, we're honestly talking about this as an epic strategy, and thus only epic characters will be considered.
By a brace of wands, i henceforth mean some sufficient number of backup wands such that all item daily power uses conceivably gained either from milestones or from resting could be funneled through them.
Playing the cash game
Collecting a brace of wands relies on the extreme difference in price between items as you go up in tiers. As level-appropriate items are quite expensive for your wealth at those levels, you can't afford to build a brace of wands with the best items you could possibly build. You can, however, afford to build them in a lower tier. This is why we care about Mesmeric Hold and Force Volley - they make up the difference in tier with an inherent bonus to hit.
A wand of Mesmeric Hold is at least a level 18 item (+4), and thus relatively cheap for a 21st+ level character. Cheap enough you'll barely notice the dent they make on your pocketbook. (< .5% of total wealth collected by a 21st level character, to give you some idea.)
A wand of Force Volley is at least a 23rd level item (+5), and thus pretty cheap for a 26th+ level character.
As both provide a +4 to hit single targets, they more than make up the difference in item bonus between your primary implement and the wand.
The following section looks at exactly how much wealth a character has, but in the context of level 21, with 1.75 million gp collected (and a likely 1.2 million or more in useable gear), the 85k gp for a wand +4 of Mesmeric Hold doesn't seem at all problematic to acquire, but 10 of them may be problematic. By 26th level you might have a wand or two of Force Volley, but 10 wands of Mesmeric Hold wouldn't be overly burdening (~10% of your wealth).
Expected Wealth per level
all values are in gp
[spoiler]
Level | Value | E(Party) | E(Ind) | Ind Wealth
1 360 3760 752 752
2 520 5360 1072 1824
3 680 7600 1520 3344
4 840 10480 2096 5440
5 1000 14000 2800 8240
6 1800 18800 3760 12000
7 2600 26800 5360 17360
8 3400 38000 7600 24960
9 4200 52400 10480 35440
10 5000 70000 14000 49440
11 9000 94000 18800 68240
12 13000 134000 26800 95040
13 17000 190000 38000 133040
14 21000 262000 52400 185440
15 25000 350000 70000 255440
16 45000 470000 94000 349440
17 65000 670000 134000 483440
18 85000 950000 190000 673440
19 105000 1310000 262000 935440
20 125000 1750000 350000 1285440
21 225000 2350000 470000 1755440
22 325000 3350000 670000 2425440
23 425000 4750000 950000 3375440
24 525000 6550000 1310000 4685440
25 625000 8750000 1750000 6435440
26 1125000 11750000 2350000 8785440
27 1625000 14250000 2850000 11635440
28 2125000 16250000 3250000 14885440
29 2625000 17750000 3550000 18435440
30 3125000 18750000 3750000 22185440[/spoiler]
Value: Value of a level N item
E(Party): Expected party wealth gained/level
E(Ind): Expected individual wealth gained/level
Ind Wealth: Cumulative individual wealth
Numbers based explicitly on DMG guidelines. Each level the party collects 4 items of level = party level +1, +2, +3, +4, and cash of item level=party level x2. Thus we sum the cash values of all those together, and divide by 5 to find individual wealth collected.
Now, we can probably assume some amount of wealth is lost over time because the game incorporates a severe difference engine which acts to reduce party wealth - that is, selling/uncreating items only nets 20% of their value. However, given the extreme rate at which item values increase, this is unlikely to be a substantial fraction of total wealth - likely less than 4 million of the level 30 character's 22 million gp, assuming the DM actually follows the DMG guidelines and attempts to provide the party with items they will find useful (and doesn't do so at the expense of other items they possess). The farther away from that wealth total the characters get, the less likely they'll be hitting the presumed power levels of characters against monsters in the MM. (That the rules makes extreme deviation from those numbers possible, and the extent to which they'll do so is in no way regulated by the rules, is a huge balance problem the game never addresses, and will not be considered further here).
How much endurance?
An epic character receives 3 daily item power uses, +1/milestone. If we assume an average encounter requires 5 encounter (or better) powers (not counting utilities), it means a character without back-up wands can run 4 encounters in a day. A character with a brace of wands can instead make *13* under these assumptions. (4+3 daily item = 7 = 3 milestones = +3 = 10 = 2 more milestones = 12 = 1 more milestone = 13 encounters).
Now, some encounters are going to require more dedicated resource expenditure, and every extra power expended is 1/2 a power not gained, but even under such conditions we're likely talking 7 or 9 encounters instead of 4. That's a huge difference in encounter endurance.
Generalizing
This tactic can be applied with some success at any level, as an item merely one step lower than your main implement is still reasonably viable, and thus any power could be used at slightly reduced effectiveness. However, for the full brace effect you will want to be operating 2 steps down, and the difference will become more and more noticeable as that happens. The +4 ensures that even 2 steps removed (+4 vs. +6 item bonus) is not especially problematic.
Credits
This post owes some debt to TreantMonkLvl20, who made a mention elsewhere about some useful encounter powers a wizard has which have an inherent +4 to hit in the context of wands. It also owes some debt to the same discussions dithering about what a reasonable expectation of combat length and daily party endurance should be.
Do we have sblocks/hidden text? How do i use them? Also, help formatting the table would be nice