Man, any guy that just wants you to play root under his chords is probably not worth your time. If you're just learning, take all the experience you can get, but anyone who is musically inclined to really create something special (in essence, the feeling one gets when jamming with others and everyone just looks at each other and KNOWS something's happening but no one can really name it) will appreciate a bassist who goes way beyond the root.
I guess really there are two things a bassist can be great at: one is knowing what's happening musically enough to add fills and melodic flourishes the way a 2nd guitarist would. #2 is a bassist with RHYTHM. A bass player and drummer that are well-synched can carry any band, period.
I've jammed with enough people in my day to know that if you have one of those two qualities, you'll be playing with lots of people for lots of years to come. If you don't have them, you can easily bring a good thing down.
I play from time to time, just for encouragement, with two brothers, one being the bassist and the other the guitarist. The guitarist is awesome, I love playing with him. The bassist can have his moments, but he's brought down so many moments for me that were just about there. It's a fine line, but in my opinion, the guy saying "play A for A" doesn't appreciate all a good bass player can offer by any means.