There's a bit of misinformation here. I suggest
reading this.
Fact is that the max charge via USB is 500mA!
That's not true. 500mA is the maximum for a USB2 data connection. On a USB3 port, it's 900mA. But then there are charging ports (just power, no data connection) which can provide even more. To quote the relevant part from the wikipedia article:
"A dedicated charge port may have a rated current between 0.5 and 1.5 A. There is no upper limit for the rated current of a charging downstream port, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5 A)."
Said that, it's really recommended to use an external Charger who delivers 2A or even more.
Yes, for a tablet, a good 5V charger with 2-3A is recommended over charging via USB.
But you always have to be carefully not to "overcharge" the Tablet which will shorten the lifetime of the Batteries. Most of the chargers which you get with the Tablet didn't having a protection which cut's the charging power after the battery is full they will just continue to charge until you disconnect that charger. And on the cheap chinese batteries, the internal protection circuit is just a joke!
Charging the battery and taking care that it isn't overcharged is
not the job of an external charger! It's the job of the power management chip inside of the tablet. The external power supply provides power to the management chip, which then decides what to do with it. First priority will probably be to power the system, and then if any power is left, it'll be used to charge the battery. That's why it's a good idea to turn off a device while charging it, especially when using a relatively weak power supply. That way, all power can go to the battery and it'll be charged faster than if the system were running.
How do I know this? I've recently done some research on the power management chip used by most if not all
Ainol Amlogic tablets. That chip doesn't care whether the external power comes from an AC/DC charger or from an USB connection - with one important exception. It has a current limit setting for USB. Depending on how the system software or driver configures the chip, it will limit the current taken from a USB connection to 100mA, 500mA or 900mA (these familiar values again). But it also has an "unlimited" setting where it'll take as much as it can get. And this "unlimited" setting is the default, on my tablet at least. I've connected it to a powered USB hub and seen that it happily drew 1000mA and more over USB.
Anyway, I think most tablets nowadays support charging via USB, and with more than just 500mA (if you use a dedicated USB charger, not a USB data connection). Nevertheless, I recommend using a
good AC/DC power supply, if the tablet supports it.