I know - it's not original. And perceived wisdom goes straight to acolytes because they're cheaper, troops and have access to rock saws. This is all true. But...
At the moment, metamorphs are only a point more than acolytes, for pretty much identical stats except the metamorphs get an extra attack. Acolytes get their knife for free boosting their attacks to three each; but metamorphs have a talon as default, pushing them up to four attacks. Now, what if we spend a point to give them another talon, taking them to five attacks each, all of which hit on 2s before any auras are applied. Not bad for 9 points.
There's a trade off here - the second talon means giving up the rending claw, so we'd lose the -1AP, and the chance to rend. But weight of dice is a big deal in 40k at the moment, so does 5 attacks base make up for this?
Here's some crude maths:
5 metamorphs with dual talons - 25 attacks for 45 points, or 1.8 points per attack.
vs guard: 25 x 5/6 x 2/3 x 2/3 produces 9.3 kills (4.8 points per kill)
vs marines: 25 x 5/6 x 1/2 x 1/3 produces 3.5 kills (12.9 points per kill)
5 acolytes with knives - 15 attacks for 35 points, or 2.4 points per attack.
vs guard: 10 x 2/3 x 2/3 x 5/6 + 5 x 2/3 x 2/3 x 2/3 produces 5.2 kills (6.7 points per kill)
vs marines: 10 x 2/3 x 1/2 x 1/2 + 5 x 2/3 x 1/2 x 1/3 produces 2.3 kills (15.2 points per kill)
[I've left rending out, purely because I don't know how to factor it in to the maths here - the 10 attacks with the claw average 6 or 7 hits, which generates an average one rend, but simply adding on another kill seems too crude. if we round up to 3 kills, that gives 11.7 points per kill]
This means that metamorphs get more attacks for their points, paying less than two points per attack. Against light infantry like guardsmen they produce a lot more kills, and achieve a better points efficiency. Against heavier targets, like marines, they still produce more kills, but become potentially less points efficient than the acolytes because rending becomes increasingly powerful against thinker armour.
I've ignored various buffs and auras to keep things simple. I'm aware that the Primus would bring the acolytes up to 2+ to hit, which has less effect on the metamorphs, but you need to buy him, and have him in the right place - metamorphs can operate independently and still have their 2+. Metamorphs also gain more from a banner (in the example above, the metamorphs gain 3.5 hits to the acolytes 1.7).
So is there a place for metamorphs as a flanking, objective stealing unit - 90 points for 10 metamorphs who are throwing out 50 attacks, enough to shred any light infantry sitting on an isolated objective without being baby-sat by characters like the Primus.
What do you think?