There's been a lot of talk about Reavers lately I've noticed and I think a lot of people have realised how important AP1 is to Dark Eldar. With Reavers being one of the best platforms for Heat Lances and also bringing some backfield disruption ability these guys are the obvious answer but it does beg the question: how important is AP1? The short answer is very bloody important and let's look at why.
Dark Eldar lists will generally have 15+ darklight weapons depending upon the points level. On average this is going to cause a lot of suppression and generally kill an AV11 vehicle a turn if everything has cover which isn't much (though the mass suppression is extremely valuable). At the same time that amount of S8 lance shots will take it's toll on a vehicle based army and more often than not, lead to some wrecks and explosions along the way. However, this is not always the case and in particular for armies with little to no AP1 options (Dark Eldar, Tyranids, Orks, etc.) more likely to happen. I'm sure most players have seen a game where tanks just don't die and it's a shake-fest all round. It happens. It's part of the problem when you build lists around just ranged shooting and minimal melta-weapons for Imperial armies and it's part of the problem for Xenos armies in army design. AP1 helps the statistics of actually popping tanks and moves the game forward in this regard.
Now this could be said to be a problem of the vehicle damage table and an issue which needs to be fixed for 6th edition. It's been discussed before to varying degrees of success here, here and here but this doesn't help us with our armies now.
During army design it becomes imperative that you place a premium on AP1. For some armies like Imperial Guard, Space Wolves, Blood Angels and Space Marines, this is really easy to consider as they all have access to melta-weapons in multiple units across multiple Force Organisation slots. Importantly these units are not always suicide scoring units (like Chaos Space Marines) which means less pressure is put on the army to kill tanks and maintain a scoring presence. Other armies however don't have this luxury. Grey Knights (non-Coteaz builds), Dark Eldar and Tyranids all have to work much harder to access AP1 and make it fit within their army lists. And this is where list design becomes important. You cannot just throw AP1 into lists as it is often run in a particular way (i.e. melta is short-ranged). AP1 is significantly important but if your army isn't going to be able to take advantage of it, it's an option you're going to have to forego.
I think this is a problem I see in a lot of the previously mentioned armies which have access to more AP1 options such as Imperial Guard. A lot of players divvy up their suppression and anti-tank roles so that suppression is all ranged and true anti-tank is all close-ranged. This in itself a problem as higher AVs can protect lower AVs at range and then retreat when the true anti-tank rolls into range. This further emphasises the importance of making sure your army works as a synergistic whole. You may have awesome AP1 options in lots of meltaguns but make sure all of your ranged firepower isn't based around the autocannon for example.
In the end AP1 is a highly important aspect for all armies but isn't always a feasible inclusion. Reavers in Dark Eldar have a very high opportunity cost for example with the excellent units of Scourges and Beastmasters sharing the same slot. If you are able to include AP1 in your army in a way that fits and works within the army construct, you'll find a lot less games where parking lots just shake each other every turn and more actual damage happening. If you are unable to include AP1 in your army effectively however, ensure you have a large amount of high strength shots on multiple targets elsewhere. Sometimes you'll have those games where you can't wreck a tank but you'll at least be able to suppress multiple targets with a certain degree of reliability until the dice winds change.
