they're not useless. someone had to take care of Simba, he'd be dead without them. besides, Timon never really means it. in the end they're always best buds.
actually, among the younger fans Timon and Pumbaa are the crowd favorite. they're funny, belching, farting, cartoon characters that despite everything else still seem to teach the value of friendship.
and actually Pumbaa did help. he carried Simba to water, provided a distraction for the hyenas, totally kicked the hyenas' butts, kept an eye on Kiara (well he tried), and carried Simba back to Pride Rock.