It plays in because you cannot cast to smart devices anymore with YTM, without a paid subscription. There's also no offline play without downloading the music itself. Not to mention the UI is complete fucking garbage. GPM is an app I use 8+ hours a day during the week and 2+ hours a week on weekends.
It also suggests that they likely will kill Play Movies, since they offer your uploads on YouTube as well. Though I don't know if you can cast YouTube to your devices or not...but I know you can Play Movies.
I understand the economics behind it, there's far more likely that more people use YouTube to listen to music now, than people who buy music through Google Play Music. I'm not sure how smart the idea is to rob Paying Peter to pay Poor Paul, but I'm looking to jump ship along with a fuckton of other people, judging by the Google Play Music reddit community and other articles.