That's very true Ren, you won't pick up on 'allegro', 'ralentando', 'mezo forte' etc. from tabs, you also won't pick up any timing - crochets, semi crochets, triplets etc. When it comes to working out music that you have never played or heard before you need to be able to read notation in oprder to be able to play as the composer intended. If you only intend to play what you can already hear - eg a Randy Rhoads solo, you can listen to cd and then work it out from tab. It definitely the fastest way of working music out but there are merits to reading notation that go beyond playing popular music - mostly if you intend to play classical.

Edit - even though I know very little about theory when it comes to guitar I can actually site read music for piano. When I was 12 I reached grade 5 on piano but to my horror now I gave it up because I found scales and Bach/Mozart/Beethoven so boring to play - never learnt anything fun at those lessons... wish I hadn't given up now though :(