I don't feel any added tension in my thumb when I do it, as a matter of fact I feel like there's less tension overall and I'm able to be more relaxed when picking. Benson seems to bend his thumb back quite a bit while Adam Rogers keeps his thumb straight and his forefinger relatively straight as well and when I try to emulate that I feel really loose and relaxed while still being accurate.
I do feel like it has some distinct advantages at least for me, with the kind of pick I currently use (Dunlop JazzTone 204) if I use the "traditional" angle of the pick then the pick just kinda rolls over the string and it's harder to make distinct notes as I have to move the pick farther or even lift it up away from the strings to get a good note. With the kind of angle that Adam uses I am able to let the pick just cut through the string and rest on the string below without much effort and it helps with more distinct note seperation. Perhaps with other kinds of picks there might not be as big of a difference.
And yeah of course a lot of guys pick differently and still sound amazing, it really all just boils down to preference. Mike Moreno also has a peculiar picking technique as he holds the pick with 3 fingers and he sounds amazing doing it.