Disk images are great, but there are two reasons OSX doesn't "put everything else inside a DMG":
- Performance: you're adding a layer of indirection, since the dmg is a file on the "real" filesystem, reading or writing a file to it requires altering a file on another file system. This is not very fast; and while it may be "fast enough" for some things, it won't be for others.
- Reliability: if a disk gets a bad sector, the worst case scenario is that the data on that sector is lost. If a DMG file gets corrupted, that whole "disk" is damaged, often beyond repair.
In short, using disk images for development tasks is smart (you're keeping stuff in source control, and backups of that anyhow, right? Right?! :D), there's a lot of things it wouldn't be smart for.
Ramblings and references
The Code that can be seen is not the true Code
I haven't found a problem yet that can't be solved by a well-placed trebuchet