Because you can't check the pressure with the bicycle pump and who wants to pump a tire by hand when you can use your air compressor. I think you missed the point.
Starting with one the " spokes" on either side of the air valve: cut every fourth( or seventh) spoke out. Grind , file , sand the nubs smooth with the rim. Powdercoat or your choice of finish on the wheel. You n ow have a functional custom rim.
You'll be glad you did. It's so much easier to check/add air. Here's a pic of it on the front wheel.
I realize this is a rather old post, but just thought I would add this for anyone of thinking of going to angled stems. Couple of years ago I bought a pair of high quality 90 deg stems with the plan of having them installed when it became new tire time. The HD dealer in Dothan, AL put on the new tires, but refused to install my stems ...... claiming liability issues.