"Do not award XP for creatures that enemies summon or otherwise add to their forces with magic powers."
As written this rule is awkward, I understand where it's coming from as a summon spell is not really much different to a fireball or whatever other spell. However as written it would suggest that constructs and many undead should never be worth XP; they do however make a bigger difference to an encounter than a single fireball (eg).
It also, if taken to it's obvious conclusion would suggest that anything the summoned or created creature kills gains XP for the creator/summoner. So logically a dungeon where the wizard lives could have 10 (example number, to make the point) stone golems, which have been killing intruders and possibly even being sent out to kill other creatures; thus giving the wizard XP while he sits around doing nothing except creating other golems when appropriate (in mechanic terms, when he's got the XP).
I'm guessing that most if not all of you only apply this rule sometimes, I'm just wondering what your personal versions are.