Apart from Forad Darre and Tamur Leng, who have to be taken into the party at different points in the game, the one I found most useful was Edina A'Mor. There's a bug in the Learning skill which hugely inflates the XP you get, and makes training very expensive. This is compensated for by a bug in the banker hireling; with her in your party, all you have to do is to repeatedly put money into the bank and take it out again, and watch it build up. Dismiss her when finished.

The next best use is to have a hireling in your party to divert attacks which would otherwise be made on your player characters. Initially, Forad Darre is a useful fighter, but your characters will fairly quickly reach a level where they become better than him.

HTH!