A bit of a balance issue - not in terms of combat difficulty, but in terms of balancing Fun(lots of enemies) with Fun(playable). The more creatures/things/people/*ahem*horses*ahem* in an area, the slower the framerate for those with slower computers.
And even for my lovely little beauty *pats his CPU affectionately*, a large (read: 9 or more) creature spawn will slow things down for a moment. If you spawn a dozen or so zombies ALONG with the bodaks/risen dead/etc., you run the risk of spawn-related lag deaths to the deadlier adversaries. And you know if you've got a cleric in the party (and who wouldn't, if you're going crypt-crawling), that they'd last all of about three seconds until said cleric hit the big "turn" button in their quickbar.
So I don't really see overall benefit (save for that moment of exploding undead) being worth the boost to lag.