I kinda dig the usage of white girl vocals

Are they literally sung by white girls? Who knows. But they are definitely white-girlish

Two obvious ones I think of are "Hands on the Wheel" by Schoolboy Q
http://www.youtube.com/watch?v=KWlu8VhcqtM

and "The Recipe" by Dre and Kendrick
http://www.youtube.com/watch?v=dOsRe2MTofo

How big is this trend really?

Is it new or has it been going on for a while?

With new ways of marrying electronica to rap being explored and white girl chic becoming the most relevant it's been in rap since Blondie I would assume this is a byproduct of other trends

or are people just sick of more traditional r&b hooks?

I mean r&b kinda sucks these days anyway but what happened to the mary j's on "all I need" type hooks?