I think the point is that people using underblankets have realised that they sleep comfortably with their sleeping bags over them in quilt mode. Therefore the logical step is to make a narrower (and therefore lighter) quilt for hammock use.
If you are using the underblanket, you do not need a mat in your sleepingbag. You also do not need another layer of useless nylon under you. You have two layers of nylon with the underblanket, as well as the nylon body of the hammock.
The criticism of the macpac (and similar) bags was that drafts were felt, where the insulated part of the sleepingbag met the sleeve for the mat. The neve had a down filled baffle installed to combat this, but this added more weight.
With quilts, if you get a draft, you tuck the quilt in. It's that simple. If you get a draft in a top-bag like the neve, you're buggered.
The other point is ease of use. With a sleepingbag (or top bag) you would have to get into it, zip it up and then position yourself in the hammock. If it were a top bag, you would have to ensure that the insulated side of it did not rotate round during the night. With a quilt, you put it ontop of you while lying in a hammock.
I think there are already quite a number of top bags on the market for those who wish to sleep on the ground (macpac, Rab, gossamer gear etc.)
The quilt that Ian has designed will be lighter than carrying a sleepingbag and more compact. It is also specifically designed for hammock use.