In general, I think it would be most optimal if technical decisions didn't hinge on legal restrictions. GPL makes this less likely because of its absolutionist stance.
In this case, it's a mutual incompatibility. This is just as much about the CDDL's restrictions as it is the GPL. The CDDL was created after the GPL, and it was an intended incompatibility. The GPL could have been the MIT license and the CDDL might have included a "cannot be distributed with MIT-licensed software" clause.
The point is that now that nobody has an interest in keeping ZFS out of Linux, it's time to relicense! That's all there is to it.
Of course, you can't, which is why you're still following me months later posting impotent comments. I'm staggered how deep this bit, I really must have hit a nerve.