I really find that it gives you an added flexibility. I still prefer to do Aikido to anything, but sometimes people don't attack you in a manner that you can utilize effectively with Aikido. So, I've been supplementing my Aikido techniques with Judo sweeps, etc.
I think I've heard somewhere that Ueshiba used sweeps sometimes for kuzushi. I sometimes use them too, but usually to cover up holes in my technique
Regardless, in my opinion, we don't live in a time where in order to study from a master you had to live in his dojo, practice from dusk to dawn, and he controlled your life in every way imaginable. You study Aikido in your spare time, and you're the only master of what you do with your spare time.
If you do choose to cross train or spar, I think It's a good idea to ask for your teacher's opinion or even approval. After all, you are his student free willingly, you probably have respect for his knowledge and judgment.