I agree with a lot of what has been said here. Although, ideally, I don't want to go to the ground (and I train hard to not be taken down), I have trained in BJJ just in case I am taken down. Being that a fight is a very fluid situation, I think you should be trained for anything that may happen. No reason to not be well-rounded and at least take a few classes here and there in other styles, just in case.
Whether it should be taught in Aikido classes is not up to me, but I remember when taking TKD that we used to have a "guest" instructor a few times a month that would come in and teach other martial arts; my first experience with Aikido actually. The same thing occurred in my BJJ class, we used to have a guest Judo instructor come in once in a while. In fairness though these were both 5 day a week training schedules, which is not the case in most Aikido schools. If you are only already training 2 days a week in a style, I can't imagine having that replaced a few days a month with another style. I tend to pick the schools that train more.
The problem with it being taught in Aikido dojos has to do with how the instructor was trained. They can't teach it if they weren't taught it. So it is kind of a trickle down effect, and then you get, what I think you all call, aiki-bunny?