Hasn't this topic been discussed at length?
I use one alt, but I feel having two alts should be max. It's silly seeing a line of alts following someone's main. Initially my alt was a simple gatherer, because I didn't have a ton of time to play and it was my way to maximize the time I did get to spend playing. That alt now has pretty much become a full combat player and something that is a perfect compliment to my skiller. The main problem with RS was the aspect of sitting at a tree and chopping, or standing by the water and fishing as you stare at the screen or watch something online. Well, I don't care for many shows at all, so I choose to have a second player going to keep me interested.
The second part to this is people's competitive nature. Before highscores went down, I had one or two skills at number one - but I couldn't care less if I got bumped. I don't play for the rankings, therefore the alts and perceived advantages don't bother me. However, for those that are competitve on the highscores - I say adapt or be crushed
(Too lazy to look up right now, but I know there was a poll concerning alts - check out the results to see what the community feels)