The current rating of a charger is what it's capable of supplying, not what it will supply all the time. The phone has a current limiter (bionic's is at 850 mA). So even someone using a 2.1A USB port in their car will only draw .85A into the phone. USB ports on computers, for some perspective, are only capable of supplying 500mA of current, which is why your phone charges faster from a wall or car charger than it does from your computer.
I know it shouldn't matter b/c of the current limiter, but I have seen a huge difference in charging times when using the 2.1A slot compared to the 1.0A and lower options. I had previously been using this 950 mA Moto charger and would arrive to my destination with either the same, or a lower charge than when I set out. Meaning that the charger could only keep up w/ the usage of the phone, but never overcome it. I tested this with the same battery, at approximately the same battery levels, running Navigation and PowerAmp for my commute home for a week. Then I did another week's worth of commuting testing using the 2.1A adapter, same parameters. I can easily go from 60-70% to 90%-full in my 20-30 minute commute home. This isn't just a placebo effect, or assuming that more is better. I knew that it shouldn't matter, but it did. Take that for what its worth, my Bionic is also one of the ones that can use pretty much any micro USB cable on the planet for charging and data transfers.