Seems like tool sets skip "uncommon" sizes to reduce price... having said that, quality sets usually don't skip ANY sizes. My experience with SK, Snap-On and Wright is that sizes are NOT skipped for wrenches and sockets.
I always buy complete sets (no skipped sizes), never know when you'll need an oddball size and then it'll be hard to find later down the road (if you want to stick with the same brand you already own).
I know I've used at least each size 6-19 mm at few times on various vehicles, motorcycles, bicycles, etc... For me the least common are 6,9,11,15,16.