The old version of Disk Utility must have been pretty nasty to justify a full rewrite. I remember it getting quite unreliable at some point around 10.7 when with reading/writing disk images. Partly because the UI relied on non-obvious drag & drop / modifier key mashing to do certain things and it didn't have much error handling to clearly explain what went wrong. It wasn't even that good at explaining what operation you were about to perform. The new version (supplemented by using diskutil for advanced stuff) is an improvement in my opinion.