Thank you all for you responses. My question was not meant to imply that you should not go outside aikido...it really was to ask why?
Well, before the war Morihei Ueshiba brought instructors from Kashima Shinto Ryu into the Kobukan to teach Kisshomaru and some of the others sword.
Sometime after the war Ueshiba ran into Yoshiyuki Sagawa (the Daito-ryu instructor) and invited him to go up to hombu as an instructor. It fell through before he actually went, but the invitation was made.
Why would you go "outside" for anything? Because that person has something to offer you. Clearly Ueshiba had no problem with it, why would anybody else?