Well, as you said yourself, it would endorse spam-threads for the sake of the threads. If you want, you can take a look in the game section of the forums, and ask yourself how many of those that deserve to not be locked or not having been made at all, and try and find how many who actually deserve to get the few ap for being made (if that was so). The same with the Tavern, well, actually any section of this site has both locked and deleted threads, and open threads that is best left forgotten. It could encourage good solid OPs and well planned threads, but it would be more likely to simply end up a new way of gaining points for no reason. Besides, just look at a section like this, where many ask questions explained somewhere else. Those threads would count as well.
But to keep it to the basic reasoning, and to quote Carlie (though not exactly): "The ap system is not likely to change".
