Translation: If they skip a size, you probably don't need it anyway.
If you do, buy the missing size and add it to the set.
Far as I'm concerned, the only sizes more useless than 16mm are 11mm, and the various SAE x/32 such as 11/32. (Although, I have had recent need of an 11 mm for the first time in ~thirty years.)
11mm is common on GM bleeder screws. If you work on Domestics you'll probably use it all the time.
I would say skips in 1/2 drive make more sense but not really in 1/4 or 3/8 theres not many sizes anyways in metric.
1/4 4,4.5,5,5.5,5-15
3/8 10-19
Anything more than 19 use 1/2 drive and buy what you need. I have sets of 1/2 in metric and shallows up to 1 1/2 and I found I Really only use 21,22,24 30 (12pt), 1-1/4 for a puller. Some sizes between 10-19 in 1/2 comes in handy for suspension as well, mostly the larger sizes like 14,15,17,18,19
