It's no different to how actors do the big Hollywood films for a paycheck so they can afford to keep doing the smaller indie films that pay next to nothing.
Whether you've liked Spencers' run on X-Men or not, it's obvious he's put the effort in doing the research into the characters and their history.
He's taken small story lines from previous issues and really expanded on them as well as making reference to little character moments going back years.
There is one scene in Fallout with Rogue in a church that really stood out to me showing that he had done his homework where she is talking about the end of days and he referenced the first time she saw Angel & Night-crawler together, likening them to an Angel & Demon.
Also I can't see him deliberately doing a bad job just for money as that will hurt the numbers for people then going and picking up his creator owned stuff. The reason I checked out Morning Glories is because I was enjoying X-Men, if he'd been doing a bad job I wouldn't have bothered (which is the reason I've been reluctant to pick up Walking Dead).