New Members: Full-Fledge vs. Salaried

Many bands who made it eventually have replacements, but in almost every case, they are paid less. Sometimes someone comes in as a salaried member, playing in the studio and live, but after an album or two, they've become full fledged members. Aside from publishing, does this mean they make the same amount of money from subsequent sales?

I'm guessing it's the player who gives the band an ultimatum. Make me a member or raise my salary or I'm out.

This happened to Laudir de Oliveira. Not a single word was said about him in the Chicago documentary. They even gave some time for Donnie who was there for about 10 minutes, though, and others... He was a fulltime band member from '75 to '82, and even participated in the studio for a couple of years before attaining that status. Sad to say he passed away about six months ago, only the second Chicago member to be gone.

Does anyone know other things behind the scenes that makes one choose one way or another?