It's a n- Just because it hasn't worked yet doesn't mean it will at some point, kinda like nuclear power, y'know.
Except the technology needed for nuclear power can improve. The technology needed for communism (ie. rewiring the brain) will not be around for the foreseeable future.
I also wonder whether communists realise how much progress is due to capitalism. Under communism there would need to be full employment, otherwise it kind of defeats the point of communism (worker's paradise and all). And when there's full employment, there's no competition and almost no progress gets made. Why would a company find a way of making steel quicker if it means that 1000 workers get made redundant?
And if everyone lives by the 'from each according to his ability, to each according to his need' creed, what happens when I want some 'luxury' item? Say I want a guitar. Ok, here's your guitar. What if I want a flute now? Can I have that? Two flutes?
So either, communism allows for every member of society to have everything they want (so, a system that assumes infinite resources, which is the very thing they berate capitalism for), or a system arises that dictates who gets what and when. So you'd either flat out not be allowed the things you want, or a huge, bloated bureaucracy arises (sound familiar?).