I really like the idea of a friendly and broad community of "Aikido" students who get along and can share experience and ideas.
I think if the seminar is organized on the onset as an experience tasting different flavours with the participants understanding that and having the maturity to balance new ideas against what their own organizatiuon and instructors teach then that is good.
One thing I feel deserves comment on is that I am not so sure Aikido groups are xenophobic as much as they struggle to maintain their own standards. Don't forget they split or set up their own organization for a reason which usually has to do with differences of opinion over training methods and what to emphasize.
I'm not so sure going to such a seminar would be good for all students until they are really grounded in the path of their own "style", etc.
I have gone to seminars and other Dojo's who follow a different school of training and I know enough to look for good ideas but also I realize if when I go back to regular training and started doing techniques "their" way I would have trouble with my Shihan. Not because he doesn't respect other styles but he has his personal ideas of the best way for his students to learn and apply Aikido.
Martial Arts still has a lot to do with the personal leadership of the Shihan.
This is not meant as a critism of cross sharing between Aikido Styles but maybe just a caution.