9 Harlequins with Kisses + a Troupe Master with a PW is 236 Points
9 Howling Banshees + Exarch with Executioner is 182 Points
9 Striking Scorpions + Exarch with Claw is 187 Points.
Which is to say, the Harlequin squad you have detailed cost 30% more than the Banshees or the Scorps do. In light of that, the fact that they kill 12.5 MEQs of 10-man MEQ squad compared to the Banshees' 9.5 seems like it's not such a great deal. Killing 19 GEQs instead of the Scorpion's 17 also seems not worth the cost increase.
It's probably worth it to field them against Plague Marines etc though.
The reason why I commented in the first place was not because I think Harlequins are more cost-efficient than Banshees at killing MEQ or more cost-efficient than scorpions at killing GEQ. However, when it was said that Harlequins have a narrower role than scorpions and banshees, I felt like saying something, because as you can see from the table I believe that it is scorpions and banshees that have the narrow role here.
Let me try one more time.
Harlequins outperform banshees in killing MEQ units AND outperform scorpions in killing GEQ units, in addition to being better than both of them against almost every other opponent.
So if you where to take ONE unit which you would use as a counter-assault unit for instance, or to kill the enemy’s super unit, would you take banshees, scorpions or harlequins in your take-all comer list? Of course what unit one prefers is up to play style and what kind of army one fields. However, if you take banshees I feel that there are a number of units out there that does not fear you, if you take scorpions the same can be said, however, the number of units that do not fear you is smaller when you take harlequins. As the units that don’t like a banshee charge also do not like a harlequin charge and the same situation exist for scorpions compared to harlequins. Apart from having more killing potential, I also feel that Harlequins bring so much more to the table with their special rules and the shadowseer.
Although your point of cost is valid, the way I see it is that by paying a bit extra for harlequins I get a unit that can probably function against every enemy unit I end up facing. I would not feel the same way if I had fielded scorpions or banshees.
I will not go into the debate about point cost etc. as I think that is a quite complex subject. For instance for banshees to be viable they need a transport, which brings their points above a maxed harlequin squad with shadowseer. Of course the transport in itself has some uses apart from carrying the banshees, however, it will not make your banshees kill more in an assault. And in certain lists, as in a footslogging list, this one wave serpent will never live long enough to deliver the banshees where they are needed.
I have to say that I find your table very confusing. Where you say x number of units killed, do you mean models in the unit, or do you actually mean units? Why have reprisals only been calculated for two units?
I do mean, of course, x number of models killed in the enemy unit. Thank you for pointing that out and I have now fixed it in my original post. And as for why I have reprisals only calculated for two units:
1: I didn’t have the time to do it.
2: To be realistic the 10 MEQ, 20 boys and 10 Plague Marines should all have a power k/claw in the unit, which means that it takes a tad bit more time to calculate the casualties from reprisals.
The point which I feel that you are overlooking is that while Harlequins are superior in terms of pure mathematics, the advantage which they have in terms of killing power compared to Howling Banshees and Striking Scorpions is not significant enough to warrant the points paid to take them. For example, the Harlequin squad you cite below, which would also need a Shadowseer, is significantly more expensive than the Striking Scorpions or Howling Banshees. Blazinghand's post illustrates this issue in more detail, so I won't go into further detail.
What if I say that these howling banshees you mention also need to take a transport to function on the battlefield? Then these banshees are more expensive than the harlequins, and you end up spending over 1/3 of the total cost of the squad on a transport, which doesn't make your unit more "killy", it only ensures they get into combat where they are needed.
However, as I said earlier, this is a much more complex subject and I do neither wish nor have time to go into this now.
Is it really, therefore, worth paying the points for the amount of additional damage inflicted against regular MEQs and GEQs? My answer to that is most definitely not. It is for this reason, they can be classified as being a niche unit in my view, since the performance enhancement is just not profound enough against regular units to warrant the additional expenditure of points.
I almost get the feeling that you are comparing the harlequins ability to fight MEQ with banshees, and their ability to fight against GEQ with scorpions. I might be mistaken, but if so that’s not fair, is it? My point is that given almost any enemy unit the harlequins will be able to deal with it, the same cannot be said for the banshees and scorpions. Therefore, I think that it is the scorpions and banshees that are the more "niche unit", while harlequins are the take-all comers. However, this is only my impression and I just wanted to state that I do not feel that harlequins have a narrower field than scorpions and banshees. If you disagree, then fine.
However, I do not have time to continuing this discussion any time soon.
I think the greatest knock against Harlequins is their inability to bring a vehicle into the fray. There's definitely something to be said about Banshees and Scorpions for this reason alone: the ability to divide and conquer/harrass the opponent on multiple fronts. Harlequins have only one point on their spear. Granted, it's a very sharp one, but it's one nonetheless.
It depends on the army you field. I can argue that it is a big bonus for me that the harlequins do not need any transport.
When it comes to them having only one very sharp spear, their speed can be quite astounding for a foot-based unit. With hit and run in the enemy's turn including move + fleet + assault in your turn they can cover quite the distance (26" on average).