Harleuins - I am really suffering with this unit as I want to take them but there is a high opporunity cost for that last 150 or so points.
1) as someone said already not only do they bring st 6 but they bring 3 shots on the move ......and pinning (torment grenade launchers anyone?) 3 strength 6 shots at bs 4 gives them a 50% probability of at least stunning a razorback, 70% against a typhoon. St 6 is really good against armour 10 - who has lots of armour 10?
2) Harlequins have very high initiative especially on the charge. Higher than a lot of close combat specialist out there ... what army has lots of armour 10 and high initiative?
3) They are very resistant to anti infantry shooting due to veil of tears so they have to be dealt with up close which nobody wants. Who has some of the most effective anti infantry shooting in the game, lots of armour 10 and very high initiative.
with the above 3 points (i haven't played them so please take with salt) I feel that harleuins could be very good against dark eldar.
4) Mobile cover - this is a big one for me. Sometime I want to put one of my units right here but there is no cover. Bing pop with grinning ones down and veil of tears bingo my infantry can go right behind them.
5) hit and run can lead to tricksies - e.g. you charge a unit with your harlies - they counter charge. This counter charge suddenly brings them into range of your archon. tricksy! You hit and run out dropping behind the combat for cover.
6) capture and control sentries - you can leave them in the back field guarding a troops choice on an objective to counter assault anyone who comes near. Incubi can do this but that can't shoot tanks while doing it and are also likely to get shot up first!
7) Don't forget they are one of our few sources of AP1 weapons
8 ) with furious charge and rending they can threaten vehicles - 20 attacks hitting on 4s (you're not going to attack something which moved 12 inches are you?) = 10 hits = 1 and 1/3 rending hits. thats and average of nearly 1 pen though I don't know the probability of penning (not same as average)
9) a nice home for a webway portal
incubi = killers of things with good armour saves and low to moderate toughness (marines of al variety) that aren't in cover and boy aren't they good at this. The reason they are so attractive is that you are likely to face marines save maybe two thirds of the time.
put em head to head against harles incubi ain't so good. Harleuins ignore difficult terrain (flip belts ignore difficult terrain so they don't need plasmas grenades unless they are attacking vehicles without the rending kiss) so don't lose initiative. they have higher initiative than incubi and will ignore their saves some of the time. Assuming harlies have kisses they will kill 3.3 incubi on the charge before the incubi strike and they don't care if they are in cover. This is ignoring the fact that they may be able to shot 4 shuriken pistols and a shrieker cannon. Ignoring the .3 the remaining 2 incubi will be able to kill .888889. If incubi incubi charge the harlies the harlies kill 2 firstly and the remaining incubi kill 2 harleuins. Next turn the harlquins kill 1.25 and the remaining incbi kill .88889. So smart money still goes on the harles.
Of course head to heads are theory only and highlight certain aspects. One of the aspects they highlight for me is how good harlies are against Dark Eldar.
Incubi will toast marine outside of cover quicker than harles and have pfp. Harles can't take a transport but can take useful shooting attacaks (shuriken pistol, fusion pistol, shrieker cannon) can do some nice tricksies.
There are plenty of uses for harles if you want to apply them but you need to think about it. It doesn't help if you have too many units like that in your army (e.g. mandrakes). You'll make more mistakes until you play A LOT of games and they become second nature. Incubi are much more straight forward in the way they play.
but as I said I haven't played them so this is just all talk and maths really.