This isn't necessarily a bad thing, mind you, as it keeps empty-pocketed dreamers off the project lists. "Ideation" is easy, but execution is another thing entirely.
Glad you are aware of this. The question is, if you couldn't create a prototype, how would you convince backers that you can pull the project off and are not just a dreamer? I don't think it's about being polished as much as it is showing that you can actually make the widget on a larger scale.
I definitely think a sketchup would be valuable. Just don't use photorealistic renderings.
"Projects must be clear about their state of development, and cannot be presented as preorders of finished products. Projects must show details (photos, videos, sketches) of their progress so far, along with a prototype demonstrating the product's current functionality. Projects must explain how the final design is likely to differ from the prototype, and include a production plan (i.e., how you're going to make it) and an estimated timeline.
No product simulations or photorealistic renderings
Technical drawings, CAD designs, sketches, and other parts of the design process are awesome and encouraged. Photorealistic renderings and simulations that could be mistaken for finished products or real events, however, are not allowed."
You could also consider using Indiegogo to gather funds that you can put towards the prototype.