Sugurbush joining the Mountain Collective didn't take long. LOL. Pretty obvious they're scared by Vail/Stowe and knew they needed to make a move.
I dont see this as much of a carrot for an eastern skier though, unless you happen to be vacationing someplace like Utah, and even then, frankly I can easily beat 6 days for $400 with other offers.
If you plan to travel to the participating resorts it certainly has appeal. There are a number of people I remember seeing on this board that said they took advantage of it when Stowe was previously a member.
The 50% off lift tickets at member resorts for passholders for no extra charge is the benefit that would more interest someone like me that doesn't want to be locked in to having to travel out west. As a Sugarbush passholder, if I decide to go to Alta or Jackson Hole, etc, I get 50% off their tickets without having to buy the Mountain Collective pass itself.