I find it very interesting that Elmokki has a well reasoned out way of deciding on how much WPF / WM you should take... but it looks only at damage.
For me, the deciding factor in choosing WM5 or WM6 is not the extra damage, but the extra swing speed.
Well, I edited the above post multiple times, but anyway.
The main comparison is between using skillpoints to both increase strength and ps and using skillpoints purely for WM. That's because one point of WM is neglible compared to one point of power strike. Judging purely by damage is stupid, since strength gives you some hp by itself, and if you happen to have spare skillpoints, it'll also let you take more IF for even more HP. Weapon master's swing speed benefit isn't something that's easy to quantify. In World of Warcraft etc damage output is purely about statistics, but here the matter isn't how much damage you can dish out to a training dummy - people can block.
The swing speed difference on reasonably fast weapons is fairly small between 111 wpf and 160ish wpf, but if it helps the player to score even like 2% more hits in, it makes WM a superior stat to PS hands down in situations where a player has to convert skillpoints to strength to get PS versus using most of those points in WM because they already have fairly high agility.
The main question I see here is wether people should get their maximum possible amount of WM or just convert those to strength and PS when possible. The answer is simple. If you don't have absurdly low PS to begin with and don't use your extra strength for anything except that one point of PS, wm is probably better if you can get 5+ of it instead.
So basically what my math says is:
21/21 level 30 build with 7 ath, 7 ps and 3 wm is definitely a viable build, but 18/21 with 7 ath, 6 ps, 7 wm, 3 if is probably better if 7 athletics is a must. You'll do very slightly less damage, but will have more hp and slightly faster swings. 21/18 is considerably (well, under 10% difference) better in terms of damage though.
PS is hands down a better stat than WM, but if you need to spend 7 skillpoints for one PS and could spend 5+ in WM instead, WM is the way to go.