Only thing is, it costs some fuel for the APU. ... right?
It costs the fuel use of APU from after engine start to a couple of mins after airborne, all to provide air conditioning to 2-3 mins between entering runway and airborne. At 130 kg an hour thats going to be a good 50kg per sector which would cost around $40-50. Thats quite a bit of profit margin for not much gain. Most airlines unless they are in a consistently hot environment will opt for just packs off. Thats excluding the increased APU maintenance cost.