^ With the few actors that I met in my life, it's more than just a jobs for money, they regard what they're doing as an art form, and themselves as artists.
Consider Denise Crosby, she likely knew that leaving TNG would result in long stretches of unemployment, but it wasn't enough for her to simply stand in the back of the set and deliver a handful of lines per episode. She wanted an opportunity to work her craft, not just show upk for a paycheck.
She only came back to the show on those occasions where her characters were given something meaty to do. If her character on Yesterday's Enterprise was to have the typical size role she had before, it's unlikely she would have taken the role.
Leonard Nimoy agreed to do TWOK because it included a juicy death scene, if all he wanted was a paycheck to play Spock again, the script largely wouldn't have mattered.
That some of the cast of Voyager bitched about the writers, especially after it turned into the Kate, Seven and Doctor show is completely understandable. There only so much you can do as an actor in a scene if you're not given lines.