Why built in mics, as a matter of interest? I'd ask the same question but without the mics if were to hijack this thread. (I prefer detachables whenever possible, in case of malfunction, and to fix or position them where I need. Also I've got some binaural headmics that I want to use more).
Most units with built-in mics also have sockets for external mics - so you have both options.
John - Sound-Link ProAudio
President - Federation Internationale des Chasseurs de Sons