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Like others have siad I think open source is the best way. I am just putting together a “pretty” prototype myself this week, I have been using paper and so on until now for a few months but I am getting ready (hopefully) to playtest it with a wider group of people and while the core essence of any game is the fun and enjoyment and not the components, I personally want to make it look and feel nice to play, not perfect but nice.
If I was to provide the art or hire an artist for the final product then I would use the pretty prototype to test out art ideas with different styles too. But this isn’t always necessary especially if you are planning on getting a publisher as the chances are they will have their own artists to do the art.
Another thing I have found that as a designer, working with bits of paper and so on for months with a game, it is actually nice for me to hold something that looks much closer to a ‘final’ version.
So I would say it is worth spending a bit of time but not too much, it is much more important to get it out their and tested and then when you are ready to be published worry about the fine art details.
