OK, just tried a small experiment with the sorpions.
Mildly spoilerish:
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[color:"#27384a"]
Went back to the catacombs to one of the hidden rooms (was too weak on first visit to kill those monsters <img src="/ubbthreads/images/graemlins/shame.gif" alt="" /> ), battled a giant spider:-
First time, killed montster unaided, xp = 3836
Second time used 1 scorpion, did not join battle myself xp = 3836
nb. - scorpion only fought while my char was in the room,
if I exited the battle stopped, started over on re-entry.
Third try with scorpion, using Aura of Command, xp = 1913
Granted this is a controlled situation & only one enemy, but I can't imagine it would change much in a 'normal' fight - a variable there I suppose would be when the scorpion wanders out of sight, not sure if that would affect xp. [/color]

ps: thanks to HandEFoot on 'spoiler' font color <img src="/ubbthreads/images/graemlins/cool.gif" alt="" />