It probably has some influence in the decision, but I don't think it is always the determinative factor.

I often use Priceline and Hotwire to book hotel rooms, and while I have received a couple of rooms obviously reserved for PL and HW guests (e.g., an otherwise nice room with a view of a brick wall or HVAC unit), the reality is that by far the majority of rooms I've booked that way have been normal. In some instances I have even been upgraded to executive floors and rooms with soaring views that were beyond what I paid for.

I guess if you're always paying rack rate you can probably count on getting a better than average room more often than a cheapnik like me who uses PL, but I don't know that there can be that much of a difference. I think something like status in SPG or HH would have far more of an impact on that.