Well, for humans it's because you'd need a really big ring to make it actually feel right. If the ring is too small, the difference in outward force created in your head and your feet will be different enough that your blood will rush to your feet and make you pass out.

For the most part, your garden variety of plants wouldn't be affected too much by the force gradient. There actually has been quite a bit of research put into this. I'm trying to find a source for this, but a few years back the Japanese burned a bunch money on this kind of research and didn't end up with anything substantial. If you do it on a smaller scale, just for the plants, it also creates the issue of extra angular momentum added to your spacecraft. This is manageable, but definitely something to take into account.

But, you're right. If you created gravity on a scale large enough for humans, it would definitely get rid of most of the problems for the plants.