How does that help if the tablet does not have a full USB port that is powered? Fiddling around with another power adaptor and cable is pretty annoying.
I don't know about you, but almost all Android tablets I've seen have one (or two.) This (rather popular low-end tablet in India): http://www.hclmetablet.com/, for example has one too. It's a regular USB port and no adapters are required.
