I'm sure UX research shows that the login button is seldom used on a particular site, therefore not that important.
Why seldom used? Because you stay logged in for a long time.
Is UX bull on this issue? Maybe, but if you are logged in for a long time, and then you come to log in when you've been logged out, it might be that you accept the lousy experience on this issue because you actually want to use the site... all that said I wouldn't care if the UX research suggested it was a good move. I wouldn't do it (on anything I owned, I would just argue against it if asked to implement it)
UX research also shows that one of the most important things is consistency. Internal consistency as well as consistency with other websites the user is familiar with.
99% of sites don't have so many features and sections that they'd need to omit some of them from the landing page. But for some reason they think replacing UI elements with screen-filling fancy animations and a lot of empty space inbetween is somehow a good thing. I never saw any upside in this.
I have a short attention span and am easily distracted, and modern UI trends are catastrophic from my perspective. I don't care about a fancy "hero area", just give me a menu bar that stays in the same place and gets me anywhere I need.
Why seldom used? Because you stay logged in for a long time.
Is UX bull on this issue? Maybe, but if you are logged in for a long time, and then you come to log in when you've been logged out, it might be that you accept the lousy experience on this issue because you actually want to use the site... all that said I wouldn't care if the UX research suggested it was a good move. I wouldn't do it (on anything I owned, I would just argue against it if asked to implement it)