Not trying to jack the Senior's thread but Gastanker is right about the additional bond chelates create, but because of that bond you don't need microbes to get food to the plant. There are many different chelating agents and they are not all created equal. Some are fucking great for microbes, others aren't so much. While you can use EDTA, Iron EDTA for example, and it won't necessarily decimate your microbes, the plant will only take up the Iron, but leave the EDTA behind... EDTA is considered a pollutant for that very reason, so while this may not pretain specifically to a potting mix that will get used for a very short amount of time, saying "chelates are good for microbes" is an oversimplification.
Let's say you add a carbon source to your soil, molasses in this case(which is also considered a chelating agent), microbes will take up the carbon (while releasing O) but need a source of N, this can be considered a carbon-nitrogen fixation. If there is no N for the microbes, then they will take it from your plant. Thus, after using molasses, you can actually use a mineral based fertilizer containing N to feed the microbes. I'd say using chelates with bio-active soil is totally possible (even using microbes in straight hydro can be possible), but it's very tricky business and for you to REALLY know what's going on in there you'd need a nice microscope, that's one reason I say just keep it simple, and just build a rhizosphere you know (w/o the need for the microscope) is going to be a great place for your beneficials.