the fact that a device will pull as much as it needs is true, but that is true only to the devices, appliances, and anything that is using the electricity, not storing it - which is the case with the battery. any electrical device uses only as much power as it needs. for example: a 55watt house light bulb will only use .5 amps, (110 volts AC) even though the circuit is wired for 15 amps max.
When it comes to cellphones, the cellphone is the device that uses the power and the battery stores the power. during charging, battery will try to pull as much in as you will give it, unless there is a limiting factor involved. a limiting factor can be a charger it self, which will supply 1.0 amps, .85 amp max, or what ever the case may be. also there may be a limiting factor built in to the phones circuitry it self that would allow only so much to go through ( i seriously doubt though)
By plugging in to 2.1A charger, the battery will try to intake all 2.1 amps,
Pro: you are charging the battery in half the time.
Con: if it doesn't destroy the battery right away, the lifespan of it and usefulness decreases dramatically.
This is called overcharging the battery, do some research on that and you will find out that overcharging the battery is never a good thing.
2.1A is not enough to destroy the battery right away, but if you would have plugged in 5 or 10 amp charger, it probably would, i'm just saying this to explain the concept.
I personally do use a 1.0A charger that i have left over from previous cellphone (touch pro 2) and your ipod charger should be ok too, but I wouldn't use anything bigger then that.
a small experiment that you can conduct which may or may not work. compare the temperatures of the battery/cellphone while it is charging on .85amp charger and 2.1amp, when it is on a bigger charger, it should get a lot hotter, and that is what destroys the battery.
As far as my knowledge goes, i have taken enough classes about electricity and electronics, and have been working in the field for several years, so i hope i was helpful enough and explained it in simple enough terms for everyone.