quickly, regular old aluminum spacers are not threaded inside, and so you use the length of bolt that will work for what you are doing. Just slide bolt through spacer. I've seen that some have diff width holes, so best to get one that fits closely to a regular width rack bolt for standard threaded frame eyelets.
Havent ever had any issues with using them.
handy thing with using your own bolt is if you need to add on a fender stay or an extra washer or whatever, it works, but I guess these would be no different. You just want to make sure the end that goes into the frame threaded hole goes all the way in, to get the max strength out of the bolt attachement point.
My worry with using spacers is that you're applying load to a long lever (the screw), stressing the eyelets and screw itself. I think the stand-offs are better because the flange sits on the surface of the fork and spreads the load over that area and not just on the threads of the eyelet.