For something that looks like it could grade MS, but shows just enough flattening on the beard and cheek to be questionable, I think grading would be worth the cost. Most collectors at that level want a slabbed coin and don't want to do it themselves and take a chance. Thus the value is more for one that is already done and with grade known. That's an opinion, not a fact, but generally what I see for coins in that price range.
The problem is more with the RPM. It would hold a lot more value if PCGS would put that on a slab, which as far as I know they won't (I could ask them as they have been known to do requests). It either gets an annoying little homemade sticker noting the RPM or has to be done by one of the other TPGs. And that is why this one isn't slabbed at this point, just don't see an option better than keeping it as is for now.
Back to the OP's question about popularity, I'd say for this one it's moderate at best. Possibly because the nice coins with the RPM are out of the price range for a lot of folks just looking to add an RPM and so there is no big surge in demand. There just aren't a lot of collectors who just have to have one, and there are enough of these out there that those folks do have one.
Anyway, in general if the coin itself is already expensive, like a 1909-S, then an RPM on it doesn't add a great deal more value.