Storytime:
When I first set up my home theatre receiver/amplifier in my basement, I was using old speakers and even older speaker wire. The speakers were your typical, boxy 3-way units which sounded boomy and not precise at all; they did have a big sound though, making them ideal for movies or playing DDR.
Anyway, I had these things wired up with stuff so flimsy it fell apart every time I connected or disconnected the speakers, and of course the sound wasn't the best. For Christmas, one of my friends got me enough 14AWG speaker wire to do my front speakers and my subwoofer. Not super-expensive stuff but a million times better than what I was using. The difference: absolutely nil from what I could hear.
20-year-old wire that had practically no substance at all was very comparable to brand new out-of-the-packing thick, juicy wiring. Enough said. B)
Fair enough, I've never had good home theatre speakers to do this same test with but I think this makes a good enough case for the vast majority of people.
I'm still using that flimsy wire on the speakers in my room. My computer speakers don't need "speaker wire" as they have built-in amps and just have a line-level connector to the computer's sound card. It's a really cheap, old cable too but my speakers sound fine to me.