I don't think it's just a matter of "I need the cash!"
I remember from the '90s, when I read interviews with him -- probably from Starlog or Star Trek: The Official Fanclub Magazine (those were the main two I used to buy, in addition to Cinescape Magazine later on) -- that Alexander Siddig was a fan of TNG even before he was cast for DS9. He made a guest-appearance on TNG in "Birthright, Part I" but never actually made an appearance with Picard himself. Maybe he sees this as a second chance to share some screen-time with Patrick Stewart.
And before someone tries to hit me with "They all said they were fans of TNG!" No. Terry Farrel said she used to watch TOS when she was a kid. She didn't say anything about TNG. Avery Brooks wasn't a Star Trek fan. Colm Meany and Michael Dorn were obviously in TNG. And I don't know the story for the rest of the cast, but I think they were non-fans. Alexander Siddig was the big TNGer.
Don't ask me to share the articles. I probably threw out the magazines themselves ~20 years ago.