I think it should take much less time to review code than to rewrite it from scratch, obviously, especially for very non-trivial parts like how to maximize utilization of the r600+ VLIW ALUs.
Then there's the legal review. If you follow the news, you'll know how much work legal review is for the OS drivers that are mostly written from a known amount of documentation. Try to imagine sifting through a few hundred thousand lines of fglrx code to determine if it contains any licensed code that mustn't be shared, uses any patents that aren't covered outside of fglrx or tells internals about the hardware which AMD wants to keep secret.
When AMD/ATI's linux strategy was formed, opening up parts of fglrx was discussed. But it was deemed easier to start from the scratch.
The result may not be as fast as fglrx, but it's cleaner, better structured code. To me, that's more important.