We've stayed at the Mercure Sapporo a few times in the past, after skiing in Niseko ..
I've not been upgraded to a suite - not once in 5 years & I was platinum for 3/5 years (silver & gold the 1st 2 years)

I'm not even sure if we got any upgrades - the rooms I received had a humidifier & it seemed like all rooms are of the same size.
There were some additional bathroom amenities such as bath salts but those could have been in all rooms.

B'fast wasn't complimentary for Platinum members until recently (2018?) and I didn't return this year, choosing to visit Tokyo instead ...
No plans to visit next year either ... if we do, we'll most likely skip Sapporo-shi & just head to Tokyo after Niseko

There was one year we stayed at the Ibis Styles Sapporo & I liked it even though it's a bit farther from the city center (the inclusive / complimentary b'fast was very much appreciated)
I guess the Mercure is more popular because it has a better location for visitors - minutes away from TANUKI KOJI (狸小路), Odori park as well as Susukino

Transportation is really convenient too in that area (the airport limo bus to Chitose is just across the street - in front of Sushi Zan-mai)