I don't have strong feelings about Ilyasova vs. MMorris. Their games are different, even though you'd nominally call both of them swings, or stretch 4's, or whatever. Marcus creates shots and Ersan doesn't; that's a big difference.
Marcus, to be more exact, creates jumpshots for himself, including a lot of long midrange shots. It is definitely valuable to be able to get a shot on a given possession, even if it's a bad shot.
I think that he would help his future on the Celtics if he could make plays for his teammates, or be more efficient with the shots he creates for himself, or of course both.
About rebounding. Ahem.
Per game numbers don't control for minutes played or team tempo.
Per 36 numbers are an improvement, but they still don't control for team tempo.
Reb %s control for those two things, which is great, but I'd suggest that you also need to separate defensive from offensive rebounding numbers. They are overlapping but distinct skillsets.
bogg, to his credit, makes that distinction, pointing out that Ilyasova's edge over Morris is entirely on the offensive board, which is an excellent observation - though I'm not sure that I'm ready to attribute all of that difference, with him, to being "use-based", rather than "ability-based".
Apart from any comparisons between these two, defreb%s of 18 or so are not very good numbers, and Morris' offreb%s are really rock-bottom.