I don't think all artists are imitators, though a lot of them are. Art, when Plato was writing this, mostly had to do with representing the natural world – artists were seen as craftspeople up until quite recently. I think that has a lot to do with Plato's conception of “art as imitation”. As Michael Kimmelman says “all art is a lie” - hence “this is not a pipe”, however there are certain artists who create things that have never been seen before - purely out of their imagination (Kandinsky, etc). Sure, you can argue that he used shapes, etc and those things already existed, hence an “imitation”. However, it's impossible to create something that cannot be perceived (otherwise it couldn't be experienced, and then it wouldn't be art anyway) and anything that can be perceived exists only in relation to other perceivable things because we only “know” anything by comparison. Art, as it exists today, is more of an idea – all art really exists that way, it remains in your head as a memory (the better the work the more heads it stays in). If you say that it's all imitation then you say that there are no new ideas (again, since all ideas exist only in relation to past ideas).
I think that's a rather thin way of viewing originality.