In my dojo our Sensei teaches us ground defense. In fact the majority of our tests require ground defense. He trains us to be able to defend ourselves against other arts if needed. Its really nice to know. After I test for my shodan in September I am going to take up some BJJ classes myself (My sensei co-teaches them) to enhance my ground technique. I know ground isn't good for multiple attackers, but in the case that I am taken down I want to be able to have a fighting chance of getting back up.
I do believe that it is good to have some working knowledge of ground defense because even if you are an excellent Aikidoka you cannot assume you will never ever be taken down.
Just my two or three cents.