I dunno....I've never really had to "practice" spinning....it's always just come fairly naturally to me, and that's going back to my teenage years. I look at indoor training as a way to maintain/build fitness when I can't ride outside. I could care less about any other aspect, since that's not what I use it for. I'm not saying that I'm right....but if I want to work on bike handling or whatever...I'll do that outside. If other people want to do things differently, more power to 'em.

As many have pointed out, they both have their pluses.....but for me, there's no question. Besides, I've had some longer trainer sessions (3+ hours) last winter where I'm sure I would've at least totally spaced out and have flown off the bike a few times if I had rollers .