This is an old thread, but since it's been re-opened I'll add my thoughts on the MIC input.
For some people the HD is their primary Audio Interface for recording... having the mic input is no different than having a MIC input on any other interface.
It's great for someone doing a single act. Carry the HD and a powered speaker... that's all you need. Setup a dual path (one for guitar, the other for the mic) and send them to the powered speaker... no need to carry a mixer.
As a multi-instrumentalist I find it never hurts to have another input. None of my additional instruments use mics but that doesn't matter... I often plug my mandolin or dobro into a direct box, then run an XLR from the DI to the mic input.... adjust trim as needed.
Added features are never a bad thing... even if you don't see the immediate value in it.