I keep spare microUSBs everywhere -- my apartment, my bag, my office, etc -- to charge my various devices. Having to keep a completely separate set of a different cable would be annoying. I think most people have plenty of spare micros that this is useful for them.
I would. I've got a decade's worth of USB 2.0, Micro USB cables laying around. So do all of my friends. If I'm out somewhere and need to charge my phone or (less likely) transfer a lot of data, I don't want to have to deal with a connector that can't help me because it isn't backwards compatible.
Nobody except everyone with existing chargers, cables, etc. Have you never gone out with a phone and realised you're low on charge and need to plug in? Backwards compatibility is a major win here.
I actually have a stack of 2 adapters on my car cable to let me use my iPhone 5. One handles conversion from the old FireWire voltage source to the more regulated ISB style, the other adapts the 30 pin to lightning.