Can USB charging (via mains adapter or from PC/Mac) of Android tablets be the main, primary method of charging? Or are there design issues? Are there design issues with tablets being charged by USB not seen with Android phones?

I saw this on another forum, what are your thoughts on this with regard to my question?
http://forums.androidcentral.com/acer-iconia-a100/112270-charge-via-usb-port.html#post1696260


  An important consideration in the design is the amount of current the
  usb connectors can handle. I figure the designers find that it is
  safer to design a separate charging and power connector because as the
  devices get larger they draw more current to operate and charge the
  battery. I think that is the reason they do that. Is there no 12 volt
  adapter for these devices? I don't own one yet, just leaning towards
  the acer 7 inch tablet.


I would prefer charging via USB (via mains or PC) when considering a tablet so that I don't have to carry around a specific charger.

Many Android phones use their micro USB port for charging - connecting the USB cable to a mains adapter or using the cable to connect to a full size USB port on a computer.

I see that some major brand Android tablet manufacturers (Motorola, Asus) as well as some smaller less know brands either use a separate power input (non micro-USB) or a dock.
The USB spec specifies that the power supplied over USB shall be ~5V DC, which is fine.

A single USB device (e.g. your tablet) may draw up to 500 mA of power in USB 2 and 900 mA of power in USB 3.  This is the issue - it's workable but not ideal.

Using a USB cable as a power connector between a tablet and the AC adapter that came with the tablet is a different story - they can do anything they want - but charging from a USB port is governed by the above limitations.