Cleats are a wear item, meant to be replaced several times during the lifetime of the pedals. They're about $20 a pair for most brands.

I don't use cleat covers, and I just replaced my Look Keo cleats after about 3000 miles. (They were due a couple of hundred miles earlier, I was pushing it some ) My usage is probably average. I don't use my road bike for in-town commuting, so it's only when I'm getting in and out of town on group rides that I'm constantly stopping at stop signs and lights. I only do a few organized tours a year (getting off my bike and walking a longer distance to a food stop, etc).

Personally, my pockets are already jammed full of enough junk that if I were to carry cleat covers, I'd have to get one of those top tube bags. Where they would be most useful IMO is in two places: very muddy surfaces (so you don't have to dig the mud out of your cleats before re-mounting), and very slippery surfaces like slate or flagstone walkways and stairs.