I have a feeling that if a poll were taken, an extremely high percentage of people would dislike the idea of linking an email account with Youtube or any other similar combination. Quite simply: people value avoiding any risk to their privacy much more than whatever slight convenience linking accounts will provide.
It's almost as if these social media companies know that there is almost unanimous dissatisfaction on the idea of linking accounts, but they figure most of those people won't be upset enough to really do anything about it. Meanwhile, by linking various social media accounts, they do better business by forcing people that may really only use one social media site often to have another web presence on another social media site.
Furthermore, I've noticed that such social media sites are quite pushy about making you link the accounts, with constant prompts at the log-in screen. On the other hand, it is rather difficult to disconnect the accounts once they've been linked. Perhaps not extremely difficult, but it's usually hidden deep enough in your preferences that many people wouldn't just stumble upon it.
It's just annoying that these companies seem to have zeroed in on a way to get what they want by pissing people off to a considerable extent, but not enough to create popular outrage.
This guy puts the concerns of many people simply and accurately.