Is it the norm? I dunno, maybe its just me, but I don't practice Aikido for any reason except for the sake of doing it. It is something I want to master. I have met people who are very passionate about what they do, and have been doing it for a long long time, and what they do is like magic; I wanna be like that. I guess in the beginning I had goals like self-defense and picking up chicks, but since I'm a nice guy and I got married, those goals kinda floated away. And before I got married I used to go to practice once, maybe twice a week (other time spent cross-training in picking up chicks) but I really wanted to become good in Aikido. Then someone I respected told me it was just about priorities.
There is a great book written by the late Furuya Sensei called KODO. In it he mentions that the practice of the Way is not a means to an end. You should practice simply because you love it.
In my opinion, those who go out to cross-train have a lack of commitment in Aikido. If one is practice another art soley to make his Aikido better, then isn't that disrespectful to the other teacher?
Regarding Ueshiba Sensei and his uchi-deshi and cross-training, did they go out and start other arts to make their Aikido better? I think they learned other arts first, and then came to Aikido. I know of some great teachers like Kenji Shimizu who was a 4th dan in Judo before he came to Aikido, and Shoji Nishio who had dans in Karate and Iaido. But these guys, like many others had studied other arts before coming to Aikido, and didn't use these arts for cross training. As I understand it, Ueshiba didn't even like it when certain uchi-deshi went out to study zen, so I can't imagine him approving of someone going out to study, let's say karate.
I think this is how any of the Ways should be practiced, not only Aikido.
By the way, Mr. Li, are you telling me that if I go out and cross-train in a third language it will help my Japanese? Would Spanish help? Andoleshite yo! Hikokimashoooooo!!! Adiosayonnara!!!!!