No, but many, many YouTube videos that are songs (either music videos or slideshow + song) have been re-uploaded and/or re-encoded many times, so often they don't have a high quality option, or if they do you're watching a 360p re-encoded in 720p. If you've ever listened to a song on YouTube that sounded like it was being played underwater, that's what happened to it!
I buy music for the sound quality, the ability to play back on any device type (xmms2 or my sansa player), and to support the artist.
Some artists share their songs on YouTube and being able to listen first makes it much easier to pull the trigger on buying the album. But I wouldn't listen to music on YouTube in preference to buying the files (or one of those shiny disks they had in the 2000s), even if the artist is making money through YouTube monetization.