Damn, I thought we finally had a sexy thread.
Good if you are in a cooler area but they wouldn't have much exposure to wind and I guess it limits the early and late sun your seedling are getting. I think trellising the sides isn't that great an idea as it would reduce the light even more.
You could also increase the warmth in your microclimate by using big rocks, increase humidty with a smallpond, etc One of the things we found is while mulching is great for protecting roots, too much will prevent your soil from warming.