I think the problem power swords (and axes) have in the current game is being 1D. Mathematically they're a step up from chainswords against most targets but even with their -3 AP they fall a long way behind thunder hammers, and even power fists given 'average' rolls. I'm planning to build a bunch more with different weapons so that as others have said I can either specialise with small units or mix things if wanted for a larger squad.
Bolter and chainswords get the job done perfectly. Relatively cheap, lots of attacks and no weapon upgrades allow for extra bodies. I've snipped the bolt pistols off my older models and replaced them with bolters, and will be building more to have a full 15 at my disposal.
vs Armoured 1-2W Infantry
Power swords do have this niche at not too high of a cost. They will come out ahead of chainswords mathematically when your target has decent armour (eg. other Marines). Howerver, as Calistarius said, if your opponent has a bunch of invul saves like storm shields then the swords aren't going to be effective and you'll likely be better off sticking with the extra chainsword attacks.
vs Big Multi-Wound Targets
This is where the chainswords and even power swords can easily feel like they tickle the enemy. The difference thunder hammers make is noticeable here even if the target has an invul save as any wound you can get through deals that flat 3 damage. The hammer is undoubtedly the king against monsters or vehicles, both in terms of damage output and points efficiency. Power fists are still nice too, they just lack that guaranteed 3D and can have a tendency to fall short when you need them most. That said, fists have the added flexibility of still being able to use a bolter so you can mix a few into a larger squad if you want to build against mixed threats without losing the ranged shots (as Aothaine and Crimson said).
My main issue so far has been how to best use the hammers. A MSU hammer squad is points efficient on paper to jump between cover and hunt key targets but if they get caught all those hammers are fragile and likely to die to small arms fire, and if they do get a charge in and kill the target there's a high chance they'll be dead in the opponent's turn. If you add chainsword bodies to act as shields they're going to be largely ineffective in melee against the targets the hammers want to go for and essentially be expensive extra single wounds, plus they make the squad footprint bigger and more likely to be in LoS for the enemy to shoot, though they would offer some protection if the squad gets counter-charged and could lend some bolter shots against nearby infantry before the charge.
The easiest thing seems to be staying mostly bolter+chainsword in a big squad with Lemartes, focus on ploughing through enemy infantry and leave big targets to other units. I tend to find that mixed weapon squads can often struggle to find their best use with some weapons wanting to go this way, others that way, and some ineffective or wasted depending on which you go for, but there is merit to mixing some fists in if you don't know what you'll be up against or want a bit more flexibility to give the choice rather than single purpose. It's more squads with something like 5 chainswords, 5 power swords and 5 hammers that can often sound versatile but then lack real purpose. Focusing mostly on chainswords to hunt infantry with a couple of fists for backup keeps their 'identity' and focus.
I will keep using hammers in different setups as I love their damage output, I just haven't decided yet if I prefer them as a MSU glass cannon or in a bigger squad to try and keep them alive longer.
Edited by Thoridon, 14 February 2018 - 07:45 AM.