I always run 3 units of 9 wyches with haemonculus (my 4th scoring unit are wracks with 2 liquifers). When my wyches stay alive after combat (2/3 of times, which makes me so proud of them), I've found that if they get the 3rd token then they're more likely to capture objectives because otherwise they'll probably run out of the table (sometimes it just happens) due to being under 50% in numbers and being sooo vulnerable to shooting (when not in cover).
Furthermore, I'll always spend some combat result wounds with wyches (saving with 4++ and FNP) in exchange for not running away from combat (because after doing that you can 1: be caught and killed; 2: be shot in the enemy turn or 3: have a chance and run before your own turn, then charge again... oh, and 4: run into another enemy unit or impassable terrain and die)
More about feeding our troops with pain... In first place, that's why we got out of Commorragh: pain is both the money and the bread for the Dark Kin (in plain words: fun and fluff).
Second reason: first token allow us -potentially- to get FnP ARMY WIDE, (it's not easy but, hands down, good reason), while second makes that, for example, our shooting troops -kabalites, one of the "worst" combat units in the book-, attack with WS 4, S4, I6, 2 A on the charge, saving with 5+ and FnP (pants down, another good reason!), and with the 3rd we become a daemon army.
Problem here is that you must kill a unit for every pain token, or at least take coven units. While the second strategy is viable it changes the way you go into battle (your list), and the first depends on the circumstances... if you play against Grey Knights forget about making Pain Tokens: those tin soldiers sell their lives at a high price, so you should better shut them up with splinter fire and take them as prisoners. Also, the competitive game (they say meta), makes it difficult to see infantry rich armies (even IG), which would make our army shine (if correctly played, as always).
So, apart from ideal circumstances as the ones I mentioned above, pain tokens must be harvested and used with care... people here have said it well already.