Eating properly is generally a better way to do that. But I get your point.

Trainers and fixed gears are a good match if you only have one bike but want to do some base stuff like Zone rides or high intensity intervals that generally don't work with fixed gears and real terrain.

That said, I'd rather do crap training outside than good training inside. My trainer is great for holding my bike up and serving as a workstand. I think I've logged a whopping 6-7 hours this winter. Good thing it was super cheap; riding inside sucks.