Well, I generally feel for the problems of other people, so when someone insults a hardworking man - who gets relatively little pay - for not working hard enough, I get upset that someone would have the nerve to do that.
Simply put, if Toady doesn't get any donation, he might have to get another job to get the money to live off of, which would slow down DF's development considerably, I imagine. That coupled with the fact that, as far as he can tell, he's working on what people want most tell me he's worthy of donations.
If it isn't what most people want, then that's the fault of the majority that it doesn't speak. Those that do tend to be rather demeaning in their complaint, so it's only natural those already here don't take kindly to them. Then the person that spoke up goes and talks about how ignorant we all are here, when really we just don't like it when people can't make a nice, civil complaint.
In addition: What is core to gameplay isn't necessarily core to programming. The Cores are, as I understand it, things on which a large number of major features depend on. One of these cores, or even an arc, might interfere with how milking would work if he implemented it now, and then he'd just have to do it again. And again. And again, until there weren't anymore interfering cores or arcs. At least, that's what would happen if he did his work the way you wanted him to, brenoink, and with a far greater number of features than just milking. Over time, the time taken up by the repeated reworkings would add up, and could potentially significantly delay completion - this is what durendal.mk3 was talking about.
You may know about programming, but there's a big difference between knowing how to write a sentence and knowing how to write a book. Sentences are to books as programming is to a game.