First of all; most new macro lens designs change focal length when they work at 1:1; otherwise the lens would be bigger .
The 180mm becomes a 120mm (ish)
The 100mm becomes a 65mm (ish)
The 60mm becomes a 38mm (ish)
This is easy to calculate adding extension tubes; for a 50mm lens you need to add 50mm extension to add 1X more magnification
The MP-E is 65mm at 1:1 and becomes a 38mm (ish) at 5:1; if the lens was 65mm all the way it would be huge
Saying that and as I macro photographer I could not care less about a macro zoom; this is the kind of lens non macro photographers would buy.
What macro photographers really need is a long due revision of the MP-E; a 0.5-5X range would be more usefull and the lens needs a round aperture and cone shaped tip (like the old Olympus 38mm bellows lens)
I know dalantech work, was one of the first macro photographers I followed but for best results focus stacking is necessary, both because difraction limits resolution and thin DOF
You can see some of my work here http://www.flickr.com/photos/seta666/
but check other people as John Hallmen (best field macro photographer I know of) and Nikola Rhame too