If I wanted to add 5v for a Pi or ABL servo, I'd do like TopJimmyCooks and use a BEC or other regulator running off of 12V so I don't need to add another AC plug.

AbuMaia, by kit makers are you referring to people like MakerFarm? The printer kit as delivered works fine on just a single 12V supply. It's adding stuff onto their printer that is driving the possible need for an additional power supply or regulator, and there's no way they can predict what people are going to try to add.