Right now WSS basically is a 2 feat, feat (requires WF: Slash).
It's a ki weapon, but the best monk ki strikes DONT work with it (Quivering palm, ToD, Stunning Blow).
Also, by about level 12 higher fist base damage pretty much makes fists a much better option, at a cost of zero feats.
I'm not saying monks should be able to get it for free or anything, but with the 2 feat cost, it's only really affordable on a deep fighter splash, or relatively pure monk. On a pure'ish monk, it just doesn't keep up with fists.
And a fighter splash the only real option, but because longswords aren't really that good compared to other weapons, it's sort of a strange choice. Unless you want to be able to use ki strikes with that monk splash, except, the best ki strikes cant be used...back to square 1.
Isn't there something you could do to make this more attractive? Longswords as mentioned are already pretty low on the list of DPS weapons (Khopeshes, Rapiers, Scimitars are all quite a bit better).
I'm not exactly sure what I have in mind, but something. Because right now, even though I run a WSS build, I do so purely for favor and to not have to recraft GS. Not because it's a very attractive option. It's a crutch.
And again, for the record, I recognize the synergy with Kensei, but that's such a narrow application, I just never see Fighter/Monk Kensei builds with WSS. If you want melee DPS, there are much better options.