Mike, us YJ guys don't need them...
as for the TJ, it needs them because the spring bucket (seat) sticks out to far and will kick the crap out of shock canisters.
I have even seen it bend just the piston. The reason it hits is because as you pivot the axle pinion up to correct for the lift changes, the shocks mount to the rear and the tabs for mounting begin to curl under the axle, this bringing the shocks closer to the tubes. Also I have read that the ride quality is improved because it brings the shocks back to a perpendicular set up and and less of an angle. I am not to sold on that reason but for the couple bucks they cost it is usually worth it.