Like heddheld said, its just going to be basic material values, color, specular values [that may or may not be correct for the software you're importing to], etc, you don't really NEED the .mtl file if you're going to be re-surface the model in another software package anyway. Some software reads .mtl files better than others, some add things on export that other software won't read on import. I've found .mtl to be a waste of energy most of the time. If you don't plan to use it, you can uncheck the "write materials" option on export, and that'll prevent it from creating an .mtl when you export.

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