Rapid development. There's probably things you'd get out of the latest Linux kernel's networking stack or video stack or btrfs driver or something, but your upgrade option is all-or-nothing, and you certainly don't to upgrade all of it. A microkernel could let you upgrade just the component you want to try a newer version of; it also means that there's a larger base of people like you testing the latest versions of each component, so the kernel improves more quickly.

Of course, this presupposes things about API compatibility/stability between microkernel components that may not be true in all microkernel designs.