There's one inconvenience with plain "list" since you can't differentiate "list foo" from "foo" using its output; you'd have to use "route list symbol" first and then do whatever you wanted to do differently ("list list foo" for example). It seemed better to leave that to the imagination of programmers, since there seem to be disagreements about it...
well, i'd suggest to forbid one-element lists as much as possible, but treat them as atoms. since pd only handles flat lists, we don't really loose features ... on the other hand it will hopefully avoid list vs. symbol/float/pointer bugs in patches ... would make it easier to write bug-free patches...
cheers ... tim