2) Spend a spell slot and get a better effect.
What the actual fuck? Low magic or not, why would you ever do that?
Summoning a large elemental >> ray of frost
Yes and no. Lets look at this at an actual game table. First of all, we have to know what the hell "low magic" even means.
If we're talking about being limited to relatively low level spells (like E6 or something), then you're looking at a trade off of blowing a feat, reserving one of your top level spells, and spending a slot for 3d6 damage or so vs just casting a spell. In most cases, I'd rather be dropping a Grease or Silent Image than doing 3d6 damage. Sure, the 3d6 is totally rad for a 0th level spell, but you get four of those a day, and I hope you don't have to Detect Magic, or anything.
If we're not talking about being limited to low level magic, then you have high level magic available. So sure, you can pull of 9d6 fire damage or an elemental with a 0th level spell slot... or you could spend your standard action
actually casting a 9th level spell.
In either case, I just can't be made to care about how much better a particular reserve feat might be compared to a 0th level spell. In a vacuum it looks nice, but in actual practice, the trade-off is ass.