The spacers ARE bad over time, but not necessarily on the axles. By adding the spacers, you're moving the weight of the vehicle outwards, putting more weight on the outer (smaller) wheel bearings. The inner (larger) bearings are made for all that weight. You will eventually (12-18 months, my guess) begin having the outer bearings start to let go. You'll know it when you hear a faint grinding sound coming from the wheels, then louder and louder, then BAM! The bearing finally overheats and collapses, sending the brake rotor wobbling, caliper damage, etc. If this happens on the highway, there's a good chance the wheel wobble will get so bad before you can stop that the lug studs will snap, your wheel will go flying, and you'll rip up the underside of your car.