The windshield definitely expands the usable temp range for us. I can put my head down beside hers and feel that there's very little air blowing on her at all. For a summer-only ride, I wouldn't worry about it, but for cold or rain, I think it's a plus.
It does complicate mounting slightly. I had to lower the stem mount a little to get enough clearance to attach the windshield. Not a big deal.
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On a related note, I contacted koolstop about the mount they have pictured on this page:
http://www.koolstop.com/bobike/index.php
They told me that it's a bobike mount for a 1 1/8" threadless stem. I'm still not entirely sure how it stays in place, but might be worth checking out for someone. I might be tempted to check it out if our daughter stays under the weight limit long enough.