Are you 100% sure you can link Youtube to a GMB profile still? The only real reason to do that, was to have your Youtube videos autopost to your G+ wall on your listing, but G+ has been slowly divorcing from GMB profiles for quite a while, with a final-final separation sometime last January or February if I remember right. Since then, G+ style brand pages are a completely different entity. I don't believe it's possible anymore to merge a Youtube account with a GMB profile, there's no social component to GMB at all anymore.
Even when it was common to merge though (say, 2015) I don't think I ever saw anyone make the claim that there were ranking benefits, aside from what could come from more user engagement on the G+ side of things.
I absolutely agree, those were the reasons to link when you could post straight to G+ and G+ would display a tab, etc. But I am positive there is still the ability to link YouTube to GMB, or more accurately to that brand account. In the GMB dashboard it even shows basic YouTube analytics that you can click on to quickly go to your channel on YouTube. When linked to the brand account, they share same name and profile image. But beyond that I am not sure if there is any SEO benefit. The only thing I can think of is it is more information for Google to better get a sense of what your business is. Kinda like using the sameas tag for structured data to tell Google what your social links are.
Up till couple weeks ago I still suggested it for this reason, going off the idea that more information for Google the better, but now I do not know if it is worth the trouble. Couple of weeks ago, probably longer, social media sites like YouTube are not even linked to anymore from local business knowledge panel; they still show up for brand knowledge panel.
I am still tempted to say it is still a good idea to link them because video content is growing and I have seen on knowledge panel displaying "Videos around the web" or something like that.
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