I think it is a negative on the AA side as you are taking something away from flyers at AA. Now United I think is the opposit, adding the ability to pay $250 and miles on cheap fares would be giving additional choices to flyers of UA that they dont currently have. The difference between an upgradable fare using miles in my experience has been around that amount most of the time anyway so it technically could be a wash but with the right marketing UA could push the fact they are giving flyers of UA more choices than they had before. We all like having more choices right? They would of course have to keep the current policy of no charge for using miles on the fares they have now (H and above I think).

I dont think it would be a bad thing other than it could be a slippery path leading to charging a "fee" for SWU usage or even a smaller fee for H fares and above (minus B and Y probably) as well.