Onto the Elites section today and there's a lot more to love than in the Troops section. It makes sense that the GK-specific detachment has limited Troops but extra Elite slots. Anyway, I'll hand over to Matt and he can explain in more detail.
There is much more to Purifiers than just Cleansing Flame though. I've already mentioned warp charge generation and because they are ML2 a few small Purifier squads really start to add dice to your warp charge pool. Also when compared to a Strike Squad they have 1 extra attack, are all Fearless and their close combat attack have Soul Blaze! All this for just 5 points more per model than a Strike Squad member. This is why you want 4 Elite slots.
However I am certainly not saying that I wouldn't run them, there are plenty of upsides to running Paladins when compared to regular termies. For example you can take 2 special weapons for every 5 models with Paladins, this means that they could go in a Land Raider and carry 2 specials into battle, whereas you would need 10 terminators to do the same. Also they are WS5 and so hit most things on a 3+, that's quite a big deal. But the biggest bonus for me is the fact that you can upgrade one Paladin to an Apothecary which gives the entire unit FNP, this can make the unit seem almost invincible against some armies.
I think that overall Paladins always seem very attractive until you start to write a list with them in, where you quickly realise just how many points you can sink into them. Just like all ground troops they suffer from the lack of mobility, I certainly don't like the idea of keeping them off the board in reserve and even with Gate of Infinity you can't risk deep striking them too close for fear of a mishap, so they just end up too far away and struggle to catch things in combat (enemy units tend to stay away from paladins for some reason), of course this is not always the case and with multiple units close to the enemy lines they can often get in but it is often a problem for a flashy expensive close combat unit such as this. Even with 2 Psycannons they are not worth the points to be used as a ranged unit, so ideally it would be good to have them in a Land Raider, but of course this makes the points problem even worse and before you know it you've spent half your army's points on one unit!
I can't say I'm looking forward to playing Purifiers in our next game (which will have already happened when this post is scheduled to go up). I already experienced them in the FAQ'd version of the old book against my Orks and Cleansing Flame proved disgustingly good. It's a shame they feel like an auto-include but I can't pretend I don't love the way they look with the white helmets!
We're around half way through the book now and I'm hoping people can see the positives despite the negativity that has surrounded this book.