Earthmover,
As an ex dispatcher (handling agent, not airline) there are many reasons IMHO....I'll repeat a couple already mentioned and hopefully add a few more
There seems to be a common misconception that drunk pax are not permitted because they may be disruptive, not because they pose a safety risk (so, if docile are not a problem).
My definition of intoxicated may be different to yours
Airline employees (station/duty managers etc), who later complain to my management about the grief/delay caused by offloading pax
Captains/Pursers, who when told "sorry, there's going to be a small delay while we find bags belonging to pax I've offloaded for being intoxicated", respond with words to the effect of "who are you to offload my pax, that is my decision"
I wish all crews had your attitude Earthmover; however employees of handling agents have to cover their arse!! Many handling agents look after dozens of airlines, which means dealing with hundreds of different crews/personalities. My line, (after getting fingers burnt) was "Capt we have a/several drunk pax at the gate, I am happy to deny boarding, but would you or a member of your crew prefer to make the decision" (or something to that effect).
Do the guys in here working for airlines have a more consistent approach (read: back up of your decisions from management)?