The way I see it, King needed to have an element of peril for Roland; be it the loss of his fingers, sanity (potentially), arthritus or the Chap in the latter novels.
But didn't we already have that with Walter? His ancient enemy, almost immortal (like Roland) lwho'd engineered so many of the pivotal moments of Roland's life (Susan, Jericho Hill, his mother's death, Jake's death). Did we really need to introduce a new character to ursurp Flagg/Walter's place? Especially one that was so poorly written?