At least Morte's good in a fight. Since he's just a skull, he's small and hard to hit - but his biting attack actually packs quite a wallop. Morte also has the ability to taunt his foes into engaging in melee combat with him, which is useful against temperamental magic-users who'd otherwise blast him with powerful spells. So because Morte quickly shows himself to be a valuable companion in battle, the Nameless One grudgingly lets him stick around. But it's only much later in his adventure that the Nameless One discovers Morte for what he really is.
If you haven't yet played through Torment, you shouldn't read any further, and you ought to find out for yourself why Morte's the way he is. And if you have succeeded in guiding the Nameless One through to his fateful confrontation with his inner demon, you'd know that Morte's been following the Nameless One not because he's a conniving skull with nothing better to do but because he's wracked by his conscience. He double-crossed the Nameless One in a former life long ago, and he's been trying to atone for it ever since. The long conversation Morte has with the Nameless One later in the game is a surprisingly touching moment between two characters who had seemed incapable of strong emotions because of their curses.
But Morte's always the trickster. Even when he fakes his death to try to weasel his way out of a major battle, you'll appreciate his unique interpretations of friendship and loyalty, and the strange brand of comic relief that he lends to Planescape: Torment.
Who was the first sidekick ever?