Before I started Aikido, I trained extensively in Ryukyu Kempo. It was a very hard transition, but the Twite-jutsu (quick joint locks) in Kempo transferred easily to the basic immobilizations of Aikido. The Kyusho-jutsu (nerve strikes) have made my atemi-waza more effective. I went to aikido because you literally just learned to destroy your opponent in Kempo.
Now im my dojo we also teach Crosstraining which I believe has helped my aikido. We do kicks, punches, standing grappling, (things that look like nikyo and kotegaeshi at times), and then also Rickson Gracie Jiu-jitsu. How this has helped my aikido is that I have learned to deal with more attacks than just the basics found in most aikido training. Mainly, because I am dealing with more combat savvy students. And Brazilian jiu-jitsu flows like aikido, its very soft and relaxed, when practiced correctly.
I believe getting a flavor of the different arts can be beneficial as long as you don't spread yourself too thin.