The important thing is that the load rating matches or exceeds that which the manufacturer specifies. 
Towing etc. should not result in the manufacturers spec being exceeded (for example, the max nose weight on the tow bar is circa 75Kg which is bugger all extra assuming the pikey wagon/trailer brakes are working correctly.) 
Totally agree ... but there are a few additions .... many folks will have 2 adults + 2 kids in the car + a boot full of stuff when towing .. so the tow bar load is additional to the "normal" weight, and has a degree of "leverage" given the position of the towbar.
In my case .. and the "worst case" scenario ... off to France so the roof box will be on and full , around 60 kgs, 2 bikes - another 20 kilos, LPG tank full - 40 kilos and then all the other crap we take .. BBQ, coldbox, awning, poles etc etc etc on the back seat as I prefer to have a heavy(ier) car and light(er) van when towing.
All this plus driving for 4-5 hours in (hopefully) warm conditions will put extra stress on the tyres, so I choose to use XL purely as a precaution ...
