I didn't see the exact damage formula, but anyway:
0 wpf (purely theoretical since minimum is 1) results in 85% damage done. Each 100 wpf points add 15% to that, so basically 100 wpf is 100%, 150 wpf is 107.5%, 200 wpf is 115% and so on. The first 100 wpf or so are cheap enough to get that it's probably worth it in many cases (compared to 2 points of power strike being roughly the same, though exact absolute power isn't comparable as easily as saying 2 PS is 16% damage)
Armor also reduces your wpf. I'm not sure if it affects weapon damage / swing speed, but I assume so. Yet if you consider the bonuses wpf grants neglible, it's probably still not worth it to stack huge wpf, since it's seriously very expensive use for stat points.
Basically as has been said before, don't take more than a couple of points of WM unless you are ranged or going for multiple profiencies. There are some extreme cases when wm beats converting to strength and getting ps may be worse. One of those is obviously when you don't have 7 points to get 3 str and 1 ps and you absolutely want damage no matter how neglible the boost is. You could also consider a level 30 18/18 player who has 0 wm and precisely 7 spare points. He'd get ~5.4% extra damage from 3 str and ps (1.48 -> 1.56 multiplier increase) and 3/5 points of static damage from strength. 6 wm would net about 50 wpf. From 110 wpf to 160 wpf the damage boost would be 7.4% (1.015 -> 1.09 multiplier increase). In this case the extra wpf will definitely win. Do note that these are both VERY extremely low benefit investments, I mean, 6 points for 7.4% damage or 7 points for 5.4% damage, 3/5 points of static damage and a few hp? It might be worth using those to get riding or shields or something instead (or it might not, depends on you!)
So yeah, rule of the thumb is, if you have 0 wm, it probably isn't worth it to raise strength just for a level of PS. If you have more extra points, getting an extra level of IF too probably makes it easily worth it though.
Oh yeah, and WaltF4 could be more or less wrong too, which would make my assumption that WM is pretty crap usually false.
EDIT: Also, yes, faster swings is more dps if you're whacking a dummy, but against a player you have to judge if faster swings scores you more hits or if it's worth it to just get more damage so those hits you DO get through hurt more. Regardless of that what also matters is how big the swings actually are, because if you kill someone with two hits anyway, getting +50% damage is pretty worthless as it'll STILL take two hits. That's pretty stupid assumption too due to speed bonuses and varying armor/IF levels anyway though.
Swing speeds don't vary that much anyway between most of the interesting cases, ie 111 wpf vs whatever you can get with WM compared to other options.