I've been talking with several other players, and I have found that some players think that Wyches are for killing. I think they are for holding, there is a subtle difference. Lets take a look at your average wych. I am discounting Combat Drugs here, as it is my opinion that you shouldn't judge a unit on the chance that you'll roll an ability that will make them better. Also not accounting for who assaults, so this may color the results a bit.
WS 4 BS 4 S 3 T 3 W 1 I 6 A 1 Ld 8 Sv 6+/4++*
*Only in CC
Lets compare her to Joe Average Space Marine
WS 4 BS 4 S 4 T 4 W 1 I 4 A 1 Ld 8 Sv 3+
Now lets put them in an arena and see who comes out.
Wych goes first, has a 50% chance of hitting, and a 50% chance of wounding. So we are at .25 Wounds per attack, with a 33% chance of the Marine failing his save. BUT she has two attacks. So for ever 4 attacks we get one wound, so two wyches should be able to wound a marine, assuming he doesn't fail his save.
Lets say he doesn't, and hit back. 50% to hit, but he hits on a 3+, so he has a 66% chance of wounding, with a 50% chance of the wych failing her invulnerable save.
Its close to a wash either way. But consider this, this is a scenario where a wych unit is fighting a Tactical Marine unit, who are dedicated non-assault troops. Their chances of actual winning a combat with anything that isn't fodder aren't great.
Now lets make this more interesting.
A unit of 5 Assault Marines with a standard Power Fist Sergeant, against 10 Wyches and a Hekatrix with an agonizer and lets give em' two razorflails.
Wyches will go first So we have 21 attacks, 3 of which are Power Weapon attacks. Taking into account razorflail rerolls. we get 12.5 hits, 1.5 of ignore armor. Okay, 4+ to wound, giving us 8.25 wounds. taking into account Razorflails, .75 of wych (hehehehe) ignore armor. Giving us about 2.5 wounds. Ouch.
Marines hit back, taking into account casualties (rounding down), 7 attacks, 3.5 of which hit, 2.331 hit, causing 1.1655 wounds.
Now what happens.
Lets say the marines fall back and reform thanks to their neat special rules. Well, lets see if they don't get stuck in while attempting to run away. Wyches will, generally, have 2 higher iniatiative, so they have a decent chance of catching the marines again, another round of combat!
Wyches go first, now we have 19 attacks, 11.5 hits, 7.75 wounds (remember, razor flails), 2.325 wounds. Yay, the marines are down to one guy, the sergeant. He hits back, 1.5 hits, Instant kill, so .75 wounds after the wyches dodge. Lets assume this kills one wych. So its 8 v 1. Wyches have won, and have held up an assault squad for two turns.
Lets throw them against a Dreadknight.
Same load out, 10 Wyches, 2 razorflails, Hekatrix with Agonizer.
Wyches go first, hitting on 5+, so out of 21 attacks, giving us a range of 8-9 (with razorflails)attacks, lets round down. 8 hits, STR3, 1.5 of which are poisoned (4+) and ignore armor. So we have .495 wounds from the Agonzier (5+ invulnerable save), leaving us 6.5 hits which wound on a 6+, giving us one wound. 2+ save, gives us .16666666667 wounds. Added along with the agaonizer we MIGHT have wounded it, lets say we have. Dreadnight hits back, (lets say he has two NEMESIS DOOOOMFISTS), so 4 attacks, so 2.6 hits, and 1.3332 wyches dead. tied combat.
Next round, 19 attacks, 7.7 hits, 1.5 of which wound on a 4+, so again .495 wounds from the agonizer,and 1-3 wounds, depending on how the razorflails hit. Lets say they do, and the 3 wounds get cut down to .5 wounds, so we have inflicted, again at best, a wound. Now he is down to two. Dreadknight hits back, again, one wound, tied combat.
As of know, we have held up a dreadknight for two whole turns with a relatively cheap unit of 10 wyches. And that is where their 4+ Dodge comes into its own. Big, scary monsters aren't much more likely to kill wyches than a marine. However, when tossed up against tougher massed infantry (think Orks, Tyranid Warriors if you are feeling mean, Death Co, etc) and they will suffer. Massed attacks will hurt wyches, who some consider to be a horde grinder, more than a monster will. But when you tie up that big scary bug or giant robot with some wyches, you ensure that they will probably stay their fighting, and free up your heavy weapons (yay Darklances.)
If my math is wrong (likely), make sure to call me out in the comments.
EDIT: I'll be doing a part 2, with assaulting wyches and different load outs, at some point.