The original intent for the "Mentor" TAG was for veterans to be able to indicate to new players their willingness to help and advice. There is people that still uses the "Mentor" TAG as it was meant to be used. But anymore these days it's used as the poor man's "Commander" TAG cause the getting that costs 300 gold.
People does use both "Commander" and "Mentor" TAG to form up a group of people to do map type events.