As people have said, the full zipper and the baffled construction of the Summerlite add weight, warmth, and convenience. With a full zipper, you can be more comfortable on 45-55 degree nights where you need some sort of insulation, but zipping yourself up into a mummy bag would be hot and sweaty.
My Summerlite (reg. length) weighs 20 ounces, not 19 ounces as claimed. However, I have owned several other items by Western Mountaineering (including a Flight vest, Flight jacket, and a Versalite Super) and they're all about 1-2.5 ounces over the claimed weights. As a Summerlite user, I think a 16 ounce mummy bag rated to 35 degrees is appealing, despite the 1/2 length zipper, but given my experiences, I doubt I would actually get one that weighs exactly 16 ounces.