I think SPG give more benefit towards Plats/Golds that book cheapest room than those who book higher category room.

for example

Room available :
Deluxe Room (112)
Premier room (72)
Executive club room (83)*
Junior suites (2)*
Executive Suite (11)*

*Club access included for these rooms

Case 1 :
A platinum member book 1 deluxe room for 3 nights
His chance to get upgrade is quite high as he have 168 rooms above his booked category that have the possibility to become his upgrade.
And even if he somehow very unlucky and the hotel is full, he still have benefit of club access.

Case 2
Another platinum member book a Junior suite, his upgrade chance is only from the 11 executive suites.
And also if he didn't get an upgrade, his plat club access benefit is not giving him any benefit as the club access already included in his room rate.