On what planet is defense an overrated quality?
Exactly. Might as well argue the other way around. Is scoring ability overrated in PGs because they can just feed their center and have him score the buckets?
Beverly is extremely underrated and would never be mentioned as a top PG simply because he doesn't average enough points. Yet his pestering defense alone definitely contributes very significantly to many of the Rockets' wins. He also shoots a higher 3FG% than Curry, has more rebounds (just) and two less turnovers than him. He scores significantly less though and has a lower FG%. He also has less assists but that is mostly due to the fact that Harden often distributes the ball for the Rockets so there is less need for Beverly to do so. Same goes for scoring really. Curry takes more than 6.5 shots more than Beverly every game.
So in rating their usefulness, you really have to take into account what their team needs 'cause otherwise you end up disregarding someone's worth based on your preference for a single stat. Overall, I'd still say Curry is the better player but it's not as clear-cut as people will make it out to be. That's why Westbrook is easily the best PG right now though because he is the complete package and contributes significantly on all fronts. His only (possible) weakness really is his consistency and sometimes his shot selection but even those have been pretty good so far this season.