I see "improving comment quality" as the public excuse they use to keep you logged in to one universal account to make tracking you for advertising purposes easier. Binding this to a G+ account allows them to report that they have record-breaking activity on G+ every month, because everybody is technically forced to be logged into G+ to do any-goddamn-thing, and so it makes G+ seem like some tremendous success and makes more people "want" to use it.

The problem is that annoying the user doesn't really make them want to use it more. I thought G+ might've had potential at first, but seeing all the desperate crap that Google's pulled over the past few months has not only pushed me away from G+, but Google as a whole.

Besides, did anybody really care about the quality of youtube comments? Nobody ever read them with the expectation of gaining any sort of insight or knowledge. Even if this change does affect youtube comment quality, it won't change anybody's expectations of the comments. The only shift will be from "this video is totes retarded" to "this video is stupid."