There's no way new games will be added day one, and so it's tough to judge a price. Ubi would get access to millions of people they wouldn't previously have access to, and that's really valuable. There's no competing Ubi service on Xbox either.
It also makes sense for Microsoft to take the hit on soaking up smaller competing services today. It's in their best interest to make sure these services don't splutter off, and if they can bring them all together and prove it works, it'll make or break the service. That's almost worth doing at any cost.
It just seems too good to be true for the current price, and a successful Ubi service at two or three dollars extra a month would be a great pilot to pitch for other publishers.