This is going to get me a load of flaming replies, but here goes:
It is irresponsible to your development as a martial artist, and for the aikido and martial arts community as a whole NOT to cross-train.
O Sensei cross trained, as did pretty much all the other biggies that I can think of who did really incredible things. Growing and learning are largely processes of synthesis.
Think of it this way, if you will. For any given technique, there are diminishing returns to practice. We all can look at true beginners and envy their ability to improve quite so quickly. Also, the speed with which we learn is largely a function of the gap in ability between ourselves and our teacher. Throw these two facts together, and you can see that it pays to study from more than one teacher, each of whom teaches two distinct sets of techniques.
On top of that, I read something not so long ago that said that in academic circles, something like half of the major breakthroughs are achieved by people trained in other fields and then started study in the field they innovate in. It's anecdotal, but I feel like the same could be said for martial arts. Each art engenders certain questions and ways of looking at the world within each of us. When we take those questions to another art, we learn. When we take that second art's questions back to our primary, we learn more.