Is it possible to run with "faster" bikes on 21/18 wheels, Yes. Certainly with road tires on the 21/18. But why have 21/18 wheels and put road tires on? You could go with 17/17 and put roadtires on, and have a Supermoto. But then off-roading through mud and sand etc. would be very difficult. Gravel and hard surfaces, it really doesn't mather what wheels or tires you have.
With the 19/17 wide rims, you can mount aggresive tires, which work well off-road, but because they are wider, also stick well on-road. And why care if the tires wear, that's what they are made for.
The point is. If you ride all types of roads in all types of conditions, and can have only 1 set of wheels, i think the 19/17 wheels are awesome.
As said before. Next bike I will definately have 2 sets of wheels. the 21/18 with knobbies for enduro/ off-road riding and a set of 19/17 with TKC80's for everything else.
That said, YMMV?
Suzuki DR 650 SE 2006