I don't really know what the differences between porting Haiku's userland to that or to Linux would be. Everyone loves to make arguments about how microkernels are so great and all, but there still isn't a major operating system or distribution with a significant number of everyday users on one. Plus, all of our same philosophical objections to using the Linux (or any other kernel) as the basis for Haiku would still apply.
Maybe even having more drivers for contemporary hardware? Also security, network stack?
The extreme modularity of Linux is absolutely one of its strengths but honestly at this point it just seems like such a modular system can never really feel as coherent and solid as a system built as a single unit from a user experience point of view.