Except you seem to be ignoring that the OP already has these Mac Pros, so no money is being wasted. You also don’t seem to realize that many hobby developers will use their computers for “regular” development, which Macs are fantastic for.
If you want to build a gaming rig, sure you’d be silly to not go with a windows machine with a gaming video card; but if you want an incredible development machine, Mac is often the way to go.