I learn to play by reading tabs, but lately learning to figure out and remember notations from the tabs. Especially if I'm using guitar magazines that shows the notes along with tabs or guitar pro.
I didn't opt for the electric gutiar exams at Yamaha, so I'm not sure about that. But I think if you take exams, they'll make you focus a little more on theory, but if not, in my case, you just learn styles, scales, techniques..etc and of course a little of theory but it's not stressed upon. Credentials will be useful anyhow, even if you realize that in the end you don't really need it for anything.
IMHO, to jam in a band, it doesn't matter whether it's notations or tabs, some bands might even want you to play by ear. It's still best to try and remember the song you're playing instead. Trains you to be a musician that doesn't cry if you're rocking it out on stage without a music stand infront of you, lol.
I feel that depending on music sheet to play the electric guitar too much(along with the genres frequently played), it might become a handicap in the future. Best is still to learn your song well.