3 good reasons why Mourner's Kaddish should be *led*

This is what I've explained to a few of the Yeshivat Hadar students:

1. Many of us who were raised Conservative or Reform are very accustomed to having someone up front (the rabbi, the cantor, a gabbai, the president of the congregation, etc.) lead Kaddish Yatom/the Mourner's Kaddish, and we expect to be "cued," and might not think of, and/or might be uncomfortable with, starting Kaddish Yatom by ourselves.  Been there, done that.
2. Many people who come to synagogue to say Kaddish cannot read the Hebrew alefbet/alphabet or can't read it quickly, even in transliteration, and rely on others to say Kaddish on their behalf.
3. Among some in the Orthodox community, women do not say Kaddish at all, and rely on men to say Kaddish on their behalf.  (I'll grant you that this gets a bit complicated in egalitarian minyanim when there's a woman leading, and is a circumstance that egalitarians can't avoid without compromising their principles.)

Bottom line:  Having someone actually lead Kaddish--with the option to lower his/her voice if a minyan participant chimes in nice and loud--as opposed to expecting people to stand up and say Kaddish by themselves, helps maximize the number of worshippers who are able and/or willing to honor their deceased loved ones.