My first sensei was against any other training, flat out banning you from training another art...
That reminds me of my first karate sensei.
He would blatantly bad-mouth each and every art, and tell us simple counters to every other art's techniques.
I've always been skeptical that Karate is the be-all and end-all to Budo.
Admittedly, I should be thankful, because I wouldn't have heard of Aikido if he hadn't bad-mouthed it