Whenever possible, I avoid umlauts or diacritical marks. In Mongolian, as in the other Altaic languages, the differences between front vowels and back vowels is of critical importance. Anyone who speaks the Mongolian language will know whether the names are pronounced in the front or the back of the mouth, and for most other readers, the marks are probably not relevant.
I don't really like this justification. I'd prefer to have the diacritical marks for my own reference, and maybe even a note to pronunciation for Mongol words and names!
