I can see where this would be good for people who are looking for the music side with the added bonus of ad-free and other things, but to anyone else it doesn't really make much sense to me. IMO they should have created an ad-free subscription around a 1.99-2.99ish range, or the option to get Google Play Music for 9.99 with ad-free added as a "freebie" bonus. It kind of seems like they're wanting to force you into using Google Play Music just because you wanted ad-free videos. And I'm willing to be there are a lot of die-hard Spotify users, or Apple Music users, who don't need Play Music. These people may be willing to pay to eliminate ads for a lower price, but at 9.99 for just that feature it just doesn't make sense.
To be honest, Ad's aren't ideal, but they're honestly not bad enough that I would sign up for a subscription service to eliminate them. Not only from youtube but pretty much anything, especially when it's free with ads already. Netflix could probably even tempt me down to a cheaper partially ad-supported version if they ever offered one.