I don't know about common...I've seen it a lot and I've also seen a lot of cons that do it other ways. It's neither in the majority nor the minority.
I personally prefer masquerades who factor both craftsmanship AND performance into the final score, but I've also competed in several who give out the awards separately but don't have any kind of rule saying you have to focus on one or the other. People tend to go all-out with both costume and presentation and then let the chips fall where they may. I do have to say that FORBIDDING people from getting scored in either craftsmanship or performance and making them choose which one is more important kind of handicaps what people are able to do, and can make for a much more boring show. Higher quantities of crappy "comedy" skits in an attempt to win performance, or higher quantities of walk-ons who are aren't allowed to do anything except runway-pose. Doesn't make for an interesting masquerade.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com