If you watch a lot of youtube video's it's worth it for just the ad-free experience alone. I actually did some math a while back and figured out based on my hourly bill rate that it's technically costing me money to not pay for YT premium (that is the time spent watching ads times my bill rate was greater than the monthly subscription cost).

You're not really, it just doesn't cost YouTube any more in terms of licensing the music for their music streaming service than it does to clear the rights for YouTube Premium, so they literally may as well include it.

A YouTube Premium lite is available in a few territories, but where it isn't that's because the music situation means it could cost single cents a month less than the full service and therefore isn't worth doing.

The Premium Lite subscription doesn’t include Music.