Originally Posted by Gummee!
Rollers are good for aerobic conditioning and perfecting your spin.
Trainers are good for making you stronger, but they promote bad technique.
Optimally, you need both.
that's why i use Kreitlers with a headwind resistance fan; resistance is variable by adjusting the size of the fans air input.
(i wouldn't use a plain old trainer; aside from the reason you listed, i find them terminally boring.)