and if you have not seen, the source code for FBX now ships with the new plugins so you can integrate it even more tightly in to your pipeline.
to answer your question though also…
text files as a way to store data… they are just data dump, all the formating is up to you, but you will have to make your own format writer and also reader to parse the .txt file, searching for strings and then figure out what to do with them…
vs.
xml, where it has a built in structure for how data is written and read, but it can be anything you want for the most part like a text file… you won’t have to start from scratch on creating scripts to read and write the data, just scripts to process and get what you want to save and load.
…
with that said, max stores animation all ready as an xml file the .anm files that it writes out for its animation merge tool , load and save animation is creating its version of an .xml format and so you could start with looking at that, as it is just a max script (in the max install directory) for doing the animation export and you can see what they are doing in there.