Well...me personally, I'm a bit of an interval freak, and well.....trainers make much more sense in that case.

Not to mention portability, better resistance, the ability to pummel yourself into a suffering pulp, and the added bonus to focus all of your energy and concentration on the effort at hand, rather than trying to keep the bike on a set of drums. I have found that to be particularly useful when going all out, as I can literally shut out everything and focus 100% on the effort. Plus, when going for long sessions, sometimes when your mind wanders, falling off isn't an issue with a trainer.