On paper, the idea of Boo's two selves becoming complete once more is a good idea. When you turn Oob into as much of a fodder character as anyone who isn't Goku or Vegeta not long afterwards, though, it's clear all potential for it went up in smoke.
He didn't really get to do anything once he merged. That's probably my biggest issue. DBZ formula dictates that a power up should at least trash a secondary villain before becoming obsolete. Majoob just got kind of schooled by Baby and then pulled a deus ex, never to be relevant again.