Alec was there because of his stage presence, vocals and looks. He could play bass well but Hueys style was a better fit for the sound. It's simple as that. We really don't know how many uncredited musicians played on all the albums, there were probably much more. Look at bands like Kiss or Aerosmith, they also had many ghost-musicians. Jon decided not to replace Alec, so he did it like the Stones in 1994...

Right now of course Hugh is a full member, due to Richies leaving etc.