You’ve got a point. Ideally, you don't want to spend anything that you weren't going to spend anyway to earn points for award travel, even though occasionally you have to spend some nominal amount like annual fees or transaction fees in pursuit of signup bonuses.
Buying points makes sense if you have a redemption in mind but you don't have the points to book it, and the redemption only makes sense where the points value for the booking is greater than the cash value of what you spent on the points.
So in my example, if I wanted to stay at a certain hotel (say the Royal Hawaiian) where a room is worth about 3.7¢ per point ($835/night with taxes or 22,500 Starpoints including taxes), I'll have "saved" some money if I bought Starpoints at 2.1¢. But with SPG, redeeming Starpoints for Free Nights includes the added benefit of the fifth night free, thus extending the value of my redemption. That means I could book 5 nights for 90,000 Starpoints (4x22,500), plus change for resort charges, for a room that would’ve cost $4,177 with taxes, bringing my redemption to 4.6¢ per point. Pretty good deal! But with SPG, you need to have the points in your account before you can book. I didn’t have 90,000 Starpoints for 4 or 5 nights. Now I do!
I don't know if it's fair to say I “saved” money (because I could always opt for cheaper accommodations), but the point is not to lose money on the deal. I could have booked the same hotel for the same dates through the UR portal with points I already have (I haven’t done the math comparing booking with UR points vs. Starpoints), but the reservation is non-refundable whereas SPG Free Nights are refundable. That flexibility has value to me. And if I decide to cancel my reservation, I’m sure I’ll use the points somewhere (although that means I’ll have cash tied up in award points).
So that’s where I’m coming from on this deal.