Harpies =
Screen. If your opponent has a lot of shooting (The BS kind not the war machine kind.) Then you can line them up in front of a unit. Their minus one to hit because of skirmisher and in general just a wasted shooting phase to kill and the unit behind them is minus two to hit.
Warmachine hunters. This on is fairly obvious. I'll explain anyway. They have an average charge distance of 18.5 which is pretty damn far. further than shades or fast cav. Now here is where they shine. They have a further charge distance than the other two but also flyers can charge over a unit as long as they can draw line of sight to what their charging. Meaning if you throw up a screen unit or something in front of a warmachine you'll screw over the shades of the fast cav. so if you mover them 17 inches away from a warmachine your still pretty likely to get the charge and any infantry that charge you will have an average charge or 11 or 12 meaning you've created a long charge for your opponent. If you choose to flee with them they never cause panic in anything in your army as well as if they die they don't cause panic tests in nearby units. 5 guys is only 55 points. you can have two units for the same price as one shade unit.
Opposing Light units = While harpies are a light unit most of the time your opponent will have a light unit and with their 2 attacks a piece and high initiative and longer charge range (+1 to combat res) they will usually win against their light units. Chameleon skinks, Dire bats, Dire wolves, chaos warhounds, pistoliers just to list a few.
That's the basics. Already worth their meager points. Right? And now the dirty!
Sandwich = Say your opponent has a large unit... It's 8th ed they probably do! You can use two units to surround the units flanks leaving your guys in a single file and exactly one inch away from them. Now that unit can only march on a leadership test and more importantly can only move directly forward or directly backwards. They can not wheel they only way they can reform is to stay in place and turn sideways in the same formation they were in or face backwards. This can create some insane tactical possibilities if used in conjunction with your other units. Say black guard.
Feigned Positioning = You can move them up like their going for the back field facing the opponents table edge usually with a small tilt towards a target unit. You buff up a nasty unit as best you can and send them forward against a point heavy target if they choose to flee as the immediate reaction then you declare another charge with the harpies more than likely catching the unit and just out right destroying the unit that is probably more than 55 points and then if you don't you can charge with the other harpy unit and catch them and even if you don't catch them you've pushed them off the board by making them flee 3 times in one turn. If this pulls both units out of position then oh well. they have to spend time and resources to get the 110 points of yours and possibly pulling them out of position as well.
Re directors = Now that chargers rank up to skirmishers facing as it would a normal unit you can make their charge angle a certain way. which if they kill the unit out right they can reform but if they have to pursue or overrun you can pull them into an undesirable position or facing.
I don't think I've ever played a game where they didn't hands down help me win. Or at least get their points backs. That being said they are most definitely a unit you have to know what your doing with or they will suck. 11 points for T3 No armor is expensive if you don't use them right.... But oh wait... Welcome to Elves!
Hope that helps you some.