If I order takeout from a place where I’d normally tip a server for eating-in, I also tip. Usually not the same 20% I’d tip for good service inside, but somewhere between 10-15%. To me, it is a service, it could be a server whose wages depend on it (and to me, the could be is important; I’d rather tip someone who technically "doesn’t need to be tipped" than not tip someone who does) so I’d rather err on the side of tipping.
I might be in the minority, but I do know that places where I do that, the staff seems to recognize my voice/name/phone number when i order and my face when I pick it up. They seem happy to see me and I always get the right stuff. Yes, everyone should get the right stuff, but I always hear how "XXX forgot this on my takeout."