I'm all for random loot chances on NPC's. Even other stuff than BP's. Firstly, it adds some "story" to random NPC's, as they won't appear as clones if they carry personal items. Secondly, as stated above, randomness in drops will have a positive effect on the amount of players.
Unpredictable drops (in addition to what they'll drop anyway) will add a nice addictive feature to the gameplay. It's exactly this feature that makes the diablo games so addictive.
Sidenote: might be a good idea to have the random item be held in the hand of te NPC, to avoid thieving.