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.
In the context of this thread, I'm mostly interested because it seems like a happy middle ground to solve your capillary action issues. However it would also be super interesting to see how a small acceleration like that could help with e.g. retinal problems from microgravity.
With NASA's current budget, they probably figure it's easier to just deal with microgravity for a spaceflight with a limited duration (like to Mars). Generating gravity only really makes sense for spaceflights of longer/unlimited duration (interstellar travel).
A capsule tethered to a second stage, or a pair of capsules is the low-cost* way to do it. The technique was demonstrated during the Gemini missions (at very low g).
*Compared to rotating space station
As to why... I suspect non-ISS life science research in orbit just has no where to get funding or support.