First and foremost, this Design Hotel soft brand allows more options for staying at an SPG hotel to earn stay/night credits AND SPG points. Second, for me, at least, when deciding between StR and Luxury Collection and the other typical luxury chains like FS, MO, Pen, etc, the points earned (towards other free hotel nights or premium airline awards) are a factor in staying at the StR or LC hotel. The same is true for Hyatt members staying at Park Hyatt, I imagine. Third, being able to use points to have an award stay is a very nice perk when I need it.
Only the 3rd issue is in question now with the SPG Design Hotels. It is possible that the award nights for SPG Design Hotels will cost too many points and/or not be available when I want them.
But that doesn't diminish that the SPG Design Hotels still gives SPG members an extra set of SPG options for the most part.
More than anything, I hope that even more Design Hotels opt to join SPG in this limited fashion, thereby expanding the SPG footprint in regions where there are no or few SPG options already. In the future, we might hope for total SPG integration, but I am more than thrilled with the ability to earn SPG stay/nights credits and SPG points towards my stays at these exciting and interesting hotel properties. The Gramercy Park Hotel in NYC is one I've wanted to try for years, and now that it's SPG that option seems to be imminent for me.