But, what if linking that info allows for better UX and features for your customers? At least for the majority. There will always be edge cases, in any feature we as developers ever build... Is it really as simple as "never do that, ever"?
In Google/YouTube/Google+ -- yeah, I think you can make an argument that people expected those accounts to be separate when they made them and G should not have forced them to link them. Bad form. But I honestly can see some cases where that sort of linking can be used positively.
In my humble opinion, the real issue is 1. How google forced the linkages, and before their latest "Pages" feature (which is hard to use even still) didn't allow for a workaround, 2. How long everyone's google and YouTube and android accounts have existed for. You made your YouTube account thinking it was separate. Now it's not, and that's an issue, due to a (correct at the time) assumption you made in the past, and that's not your fault.
It's a shame that this is such a mess. The only Google product I still use is Search these days. They lost my trust over a year ago now.