It is when you factor in armour soak and you do 2 damage vs 7 for example.
Because 1h does low damage powerstrike is important because it will significantly increase your effective damage on heavily armoured targets.
When I ran with a 24/15 build and had powerstrike 8 I had many tincans comment on how much damage I hit them for. Actually not many people regardless of their armour survived 2 hits.
Not really a big deal. With the scenario above and 15 str vs 24 you'll do about 6 more points of damage per swing even against someone in heavy armor. So, you'll probably kill them in one less swing.
So, let's say you use your hammer against some guy with 80 hit points wearing 65 armor. Let's say you have 100 wpf after armor reduction.
According to the damage calculator:
http://alpha-lider19.ru/MB/With 5 Power Strike and ignoring both speed bonus and held attacks, it will take you 5-11 body shots to kill the guy. Let's say an average of 8 hits. If it was 4-10 body shots, (which I think it really should be) that would put you at an average of 7 hits.
With 6 Power Strike, it will take you 4-9 body shots to kill the guy. An average of 6.5 hits.
With 7 Power strike, it will take you 4-8 body shots to kill the guy. An average of 6 hits.
With 8 Power Strike, it will take you 4-7 body shots to kill the guy. An average of 5.5 hits
So, if you look at this, going up one level of power strike will take you on average 1 less hit to kill a guy. The results for 5 power strike are kind of an anomaly, and I figure that the damage was just on the cusp. Doing a different value for hitpoints would probably put it in the 4-10 hit range which makes more sense.
If you added in WPF (figuring the less Power Strike the more WPF you'd have) I'd say the numbers would be even closer. Add in speed bonus, probably closer still. Power Strike just doesn't have the HUGE effect that people think it does. Obviously killing people faster is a good thing, but if that's all that matters, you're better off rolling a 2-hander or a polearm guy just because of their higher initial damage and longer reach.