Its not just vampires, but all the 'in show' demons as well...From Giles cavorting with one in latter seasons (something about a debt) to 'kitten poker' I think the writing crew wanted to show us some variety a human relation so to speak?
Maybe thats why I almost feel bad for dawnies vampire date in latter seasons...but you don't have to be a big bad to not be a big growler...the master for instance.
Though he is as wrinkled as an aged raisens butt, he's not Mr. Lion-o with the roaring and stuff- though on occasion he does do so, in fact he seems to admire the 'wickeder' side of human civilization like the assembly line! he did not rip into Buffy's neck and drain her dry, but had some intelligence behind his biting and tried to drown her instead...even in a parellel dimension he opted for some neck snapping instead of full on drain/gourge.
The roar and animal side was evident in the series even in latter seasons...almost always in Spike, also in the flunkies. I think it sereved a purpose and that purpose is sorta like an alarm or danger warning. Spike growled in latter seasons generally when he was vamping out and looking to be naughty...henchmen are naughty by nauture and are always doing it.
maybe what your keeing in on is the threat to Buffy, she handles most vamps pre season 7 with ease, back in season one it was new and dangerous. She'd hand Luke his ass in season seven without an eybrow raised...I guess what I'm saying is that in early seasons they lived long enough for you to make the distinction?
just my 2 cents.
I think the exception of growling/whatever but still being dangerous might be to demonstarte how society is society no matter the nature, be they wearing horns, fangs otr three piece suits. And you don't have to growl to be a big bad, those that have figured it out realize that.
my humble opinion.
