The benefits of becoming a lich are just too many to number: Virtual immortality as long noone destroys your phylactery, immunity to death spells, electricity, cold, poison, charm, fatigue, sleep, insanity, fear and whatnot.Cause fear,lich salient abilities like a death gaze, regeneration, mastery of necromancy, the ability to command undead whenever you please, weapon immunities, the respect of supernatural creatures everywhere
and so on and so on...
I think some designer responded to the question if you could become a lich in ToB and he said it was a great idea, but too complicated to be implemented, too bad.Mainly because it would make the protagonist fairly powerful, after all, you couldnt be killed anymore with your phlactery resting safely in your abbysal lair.Becoming a lich only offers boons, with no downsides whatsoever (well, your looks certainly wouldnt make the ladies happy, but thats what Polymorph is for
)