BC,
Having studied an Aero Eng degree, I have had a bit of exposure to the business of sideslip, so for what it's worth, will add my thoughs!
As far as I was aware, it is an airworthiness requirement for an aircraft to be able to sideslip to a certain degree. This is not so critical on smaller aircraft, but consider approaches where a wing-down technique is used - the aircraft is in a constant state of sideslip. In this condition, the aircraft must remain easily controlable for obvious reasons.
Sideslip flight is sometimes a problem for aircraft with a high degree of wing sweep. Thus typically applies to large airliners, who's swept wings (there to reduce drag at high Mach no flight) mean that a lot of aileron control has to be deflected to maintain level bank attitudes on approach. I'm no real expert, but I suspect that this is the reason that crosswind limitations are defined in the a/c operation manuals etc.
With regards to the flap limitations on smaller aircraft, then this is most probably due to the flap disturbing the airflow over the tail control surfaces leaving them short of control power. This situation is akin to T-tailed aircraft going into deep stall and not being able to recover as the "dirty" air from the stalled wings passes over the tailplane.
I have never personally come across this problem with light a/c before, but then again, I have not flown a high wing or a Cessna! All light a/c that I have flown have not been sideslip limited.
Hope this helps,
Cuban_8
U.A. - Were there any implications with the C172 regarding landing with 40Deg and kicking off the drift??