endgame POV: buffbot (totems), healbot (soso), and wiperezzer (always a shammy to pop to rez the first priest).
i am sure in TBC we may see more shammy action in instances / endgame, due to raid being smaller, and maybe a shammy doing more than buff/heal, depending on spec. anyways, thats what blizz is hoping for. its a versatile class. it can do good of everything, but not as good as specialized classes.
Casting? sure you can, but your mana pool is more limited and you drain it more easily if you go offensive.
As far as PVP goes, a shammy is a REAL pain for meh being mage. in open pvp i might get them IF i am lucky, in duels, they are tough. only way is sheep, burst, burst, interrupt/silence his heal, root, burst.
if he didn't die by then, my interrupt / silence is over and i am mashed potatoe.
On the other hand, Hunters eat you alive, even with your high armor class. a Beastmaster specced hunter should go through you without any problem, and i think a Marksman/Survival spec should get you fairly easy.
and (i dont know, havent played a lock yet) a lock should get you as well, from what i can think of pvp abilities. but then again... a lock ownz all i can think of, except someone who "surprises" the lock and bursts out insane DMG or keeps the lock controlled (feared / silenced)
alltogether, i would say a shammy is a very dangerous adversary in pvp, and hopefully more to be seen in pve.
Come and join on Starcraft 2 as well =]