But why wouldn't you want to check in online, so you can "access your elite benefits" such as not being upgraded and not getting to pick your room?!

I love how Marriott OLCI says basically, yeah, that's great, now go to the desk and beg for upgrades.
I've been auto-downgraded to rooms of lesser types than the booked room twice in the last couple weeks at big-city MR hotels on $400+/night rates. Noticed it in the stay details on the app on the day of check-in. Of course, when reminded that these were corporate rates and that I'd be happy to let my company's travel procurement folks know, the property staff acted very confused and were quick to make this "error" right...I'm sure it had nothing whatsoever to do with the fact that the properties were sold out at higher public rates...