Well for starters it looks as if per the USB specifcation that Mini-A is deprecated but Mini-B is still good. Mirco is the same width but half the thickness so that's helpful. Plus since its a newer standard and some telcom dept of the EU (I think that's who it was) decided that all new phones should use a standardized charger port (USB) that everyone should just use the micro. But I'm not sure what the status of all that is. Just heard something in the nerdy news the other day about it.
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I have wondered that myself. I think it boils down to the fact the micro USB in thinner in height and as devices get smaller and smaller, it takes up less space. Also, and this is just my opinion, I think it’s all about selling accessories at the point of purchase that are compatible with your particular model.
The biggest improvement micro has over mini is the thickness of the plug. Micro-usb plugs are significantly thinner than mini. They also can handle more power and they are can handle more connects/disconnects.
On 17 February 2009, the GSM Association announced[23] that they had agreed on a standard charger for mobile phones. The standard connector to be adopted by 17 manufacturers including Nokia, Motorola and Samsung is to be the micro-USB connector (several media reports erroneously reported this as the mini-USB). The new chargers will be much more efficient than existing chargers. Having a standard charger for all phones, means that manufacturers will no longer have to supply a charger with every new phone.
So, basically it's a standard. While it'll be more painful at first (none or few accessories will work) in the end it should be great as everything should work with everything.
It uses a smaller port and chipset which is great for all of these slim devices.
Not sure about that power rating mentioned by Anshulk though. It was my understanding that all USB was rated the same but USB 3 may be able to output more power.
The physical interface of the Micro appears to be more durable - look at the female mini USB connector, it always looks like it's about to fall off, with no real support in the male connection. The Micro looks more solid with a comparatively hefty plastic surround inside the connector to surround the interface on the phone.