It does piss me off when people say that he was the grandfather of MMA. He really wasnt. Just because he said that people need to assimilate knowledge from different martial arts doesnt make him the worlds first mixed martial artist.
If you think this you need to jump off his nuts and read your history books.
Bruce has that reputation because he was the most recent historical martial artist (preceding the UFC of course) to advocate MMA publicly to a large group of Americans. He truly was the grandfather of MMA for many American martial artists, myself included.
That being said, he was definitely not the only martial artist who did this. There are plenty of examples of MMA stylists well before his time (Kano himself for one), and I'm sure it extends much further back into history as well.
But you have to admit that Bruce did have a major impact on opening the eyes of martial artists to MMA concepts. Because he did so recently and affected quite a lot of Americans with his teachings, it is reflected in his legacy.