Wheel spacer are not a bad thing if you consider two issues:
First the centering of the wheel. Many wheels are 'hub centric' and require the center of the wheel fit the hub. If you add too much spacer then you may loose this and have rotational problems of the wheel. Most aftermarket wheels suck in this area as they are multi application and mass produced. Factory wheels seem to tollerate the loss of this better, they are stud centric.
Secondly, the length of the studs MUST be modified if the wheel is moved out a lot. As a rule of thumb the nut should engage 1.5 times the thread diameter. Thus 12mm = 18 of thread to be safe. They don't need to protrude past the nut.
All of this is a mess to consider when doing brake kits that's how I know what it's about.
And don't forget that spacers place the wheel closer to the fender if the car is lowered.
