A solid 2 (or 2.1

) amp wall wart is a good thing. The formal USB 2 spec only requires 500 mA (0.5 amp) for a PC USB port; while many of them will indeed provide more, it is an iffy, out-of-spec, thing, and what can happen is that it "works" only because the PC power supply is being over-stressed dropping the voltage -- you may only be getting 4.5V, not close to 5.
A dedicated low-cost wall wart lets you avoid the possibility of harming the PC -- at the very least blowing a port, and at worst taking out the PC power supply or motherboard -- if you have a short occur.
The plus of 2.1A is that it means you can go down to a 2.4 ohm atty or carto before you exceed the power supply spec; a 2A supply has a 2.5 ohm limit. While practically speaking a 2A rated supply will likely drive a 2.4 ohm carto fine (engineering margin), it is always preferable to stay within the
device's rating.