This is pretty common in the thriller genre, and I thought it would be common in other genres as well, but I guess not. Almost all of the authors I follow use these: Lee Child, James Rollins, Steve Berry, Lincoln & Child, Joseph Finder, Harlan Coben--all of their series books are series that can act as standalones. Sometimes, within the series there are a couple of books they recommend to read in order as they have a story arc, but not many.
This is the same thing I'm doing with my series as well. My guess is agents would be more likely to pick these up because they're not committing to a 5 book series in case it flops, but they're also practically guaranteed to get a lot of books out of the series if it doesn't. And the beauty of these is, since there is no story arc that dictates how many books will be in the series, the author can keep writing them as long as they see fit/the stories stay profitable.