Like my old manager used to say at Hardees - "If we allow employees to eat the waste, there will be more waste to eat" - Never thought I would retain a fast food manager's wisdom for a lifetime, but that has stuck.
If local affiliates have no real outlet for their subchannels, the appeal of having subchannels will dry up. They are fighting like hell for multicast must-carry precisely to guarantee carriage of all their subchannels. Why maintain 3 or 4 channels if only the 15% OTA'ers can see them. As soon as you offer to carry them, the incentive to have them jumps dramatically.
So while carrying them, or not carrying them, has no direct impact on quality - it does entice those that have them to keep them - and those that don't to consider them.