The more well rounded a player is, the better chance of them progressing.
8-Ball bad for your game? Ask Ralf Souquet, I think he would differ on that opinion. When he won, I believe it was the 2006 World 8-Ball Championships in Fujairah, UAE, in an interview he mentioned 8-Ball was actually his favorite game. I don't think it's affected his 9-Ball game at all either
Years ago I had the same narrow minded attitude about 8-Ball as your friend, because it is not quite the instant gratificationist game that 9-ball is. Over the years I came to appreciate 8-Ball, which happened more after I learned and started playing 14.1. It is a thinking game in a different way, and it varies more based on the size table than 9-ball does. When you see two excellent 8-ball players go at it on a 7' Diamond, sweet!
All games are good to know, 8-Ball, 9-Ball, 10-Ball,1-Pocket, 14.1, rotation, banks...
Personally, I croak at banks, but it's great to watch when played well.