Quoted from FrederikFrost:
If they should have been installed, they would have been installed.
I tend to agree with this, but if this drains bother someone to the point that they aren't enjoying the game, I can completely understand wanting to make some tweaks. I still don't think I could bring my self to drill into a brand new playfield, though. Fortunately, I don't have a problem with the side drains on TWD. Some of my games such as Pinbot and Tommy are much more brutal in this regard.