I don't understand buying them fully built either.
The replace-a-case that I have was perfect, since I already had $$ into a 32-spline front output, a 32-spline rear output, and a set of 4:1 gears (haven't used those, but still have them). Dimensionally, the MMW is marginally larger than the stock D300, so if you already have an existing setup the changes to install are minimal (ie: driveshafts or seat clearance). Additionally, the smaller form factor of the MMW versus an Atlas is helpful, I couldn't fit an Atlas in my buggy easily. (The Behemoth wouldn't fit either, the cam-shifting design would interfere with the passenger seat)
At the time MMW came out, Behemoth was a shitshow (they are far better now) and Atlas lead times were ridiculous.