Generally speaking, mtDNA mutates so infrequently that it tends to be more of anthropological than genealogical interest. If you have specific candidate lineages to test, and they have tested myDNA also, you may at least be able to eliminate some lines as possibilities using mtDNA.
You may wish to consider Family Finder testing. This includes everything except mtDNA and yDNA and may produce matches in -any- of your testee's lineages. It is new and the database is small as yet, so it may take some time before it pays off. Additionally, it is of limited utility if you don't already have extensive knowledge of your ancestral surnames.