When I play I use a mix of optimization and flavor. If I over-optimize and don't work on my character's personality and quirks before the campaign I always find myself being a one-trick pony. Either I will simply try to solve everything with my spells or class abilities, completely disregarding ingenuity which may have been utilized in absence of those abilities, or look toward the other characters for the same solutions.
If a character is under-optimized, I try to use my characters skills and a abilities to compensate for their lacking in other areas. For instance, if I play a rogue with a high intelligence score, naturally I will rock at a number of different skills, but this may cause the rogue to lack in other areas, like combat, because I had to use a large stat (a 16 or 18) in Intelligence instead of str, dex, or con which I would have used for combat. If I play a rogue with a lower int score but a ton of strength or dex, I may be flippier and better at dealing combat damage, but I just gave up probably all of my social skills (bluff intimidate diplomacy) so I'm more min-maxed in that I will just do damage. I really think we lose a lot of flavor on combat focus, although it makes us more effective in combat when we have to kill stuff, it takes away so much of the interaction you may be sharing with NPC's and other characters, basically it neuters the intrigue.
Of course you could probably say just the opposite, a character with multiple focuses (combat, Intrigue, Stealth) is totally possible within the system. One of my favorite classes is the incarnate. capable of filling nearly any role with a switch in melds, the incarnate is ultimately versatile. The same versatility can be found in the binder, factotum, wizard, druid, and many other classes to some extent. The only real restriction I have from playing these classes exclusively is the small headache I get when I have to pour over all of the available abilities and pick-and-choose which ones I'll need today.