the main benefit is obviously the armour weapons etc.
however its fun fighting against other people which is why many go in endlessly
as far as im aware theres no xp to be gained but them in 60 so wouldnt notice anyway
lock pets depend really, not a warlock so cant really answer which is best, though as a shaman felhunters annoyme.
as for arenas, the STV arena isnt much really, just a place to fight over a worthless chest that spawns every so often, though my guild use it as the recuitment area(pvp guild, gotta fight to get in it

)
however the arenas that came with TBC offer new armour(gladiator sets) in 2 v2 3 v3 or 5v5 arenas pitting you against similar geared opponants be they horde or alliance, its all fun fun fun for me