T3 4+ with horrible assault stats do not make a durable unit, and certainly not if you are jumping them into cover or, God forbid, into and out of cover. Dangerous terrain tests alone will whittle the unit down fast enough.
Note that we can take their assault move and "walk" them into cover; jump infantry can choose to move normally. This is far safer if we are nearer to the cover.
Against MEQs:
To hit- about 54%
To wound- about 35%
Marine Save- about 11%
Ten Drones- 1.1 (Mostly one) Dead Marines, Pinning test, they'll likely pass.
Compare to a 10-man FoF squad:
To hit- 50%
To wound- about 33%
To kill- about 11%
20 shots- 2.2 (Mostly two) Dead Marines, No test.
Drones with one 'light (which is actually a good use, as it increases their power exponentially):
To hit- 75%
To wound- 49.5%
To kill- about 16%
Ten Drones- 1.6 (more often two than one) Dead Marines, Pinning Test, they'll likely pass.
To be fair, FoF team with one 'light:
To hit- 66%
To wound- 44%
To kill- about 15%
20 shots- 3 dead Marines, Morale check, likely pass.
On offense, an FoF team deals more damage easily, doubling the Drones in kills. But what about defensive capabilities? Mathhammer breaks down more here, but we can tell quite a few things anyways.
At 12", Fire Warriors are likely to get assaulted. Even using a DF or free drones as a shield for them, the can still be hit with your opponent's entire arsenal of shots (at 12" he could shoot heavies AND rapidfire). As you've said multiple times, T3 4+ save isn't that great.
Against a squad of 9 bolters and one bolt pistol (a rarity, but still, it simplifies it):
To hit- 66%
To wound- 44%
Save- 22%
19 shots- 4 die, morale test, will probably fail due to terrible Ld.
The Drones however, have 18" range assault weapons and an assault jump. They can get out of LOS, into cover, farther away, or, in extreme cases, assault. They can also sacrifice themselves for portable cover/ assault shields.
In the first, they shall sustain little to no damage.
In the second, they sustain less damage against nearby heavy weapons, but this is only really useful if you have pinned the infantry nearby.
In the third, you can move out of the assault range of all units except for the oddities like Chaos Spawn. You are out of the rapidfire range of all units but jump infantry. If your opponent chooses to fire, they do not move (for stationary max range) and only deal two casualties.
In the final, you assault. Against MEQs, you strike simultaneously:
Drones-
To hit- 50%
To wound- 16%
To kill- about 5%
20 attacks on the charge- 1 dies
Marines
To hit- 66%
To wound- 44%
To kill- 22%
11 attacks (base plus Sarge, assuming a full unit)- about 2.5 (more two then three). The Drones will likely fall back, w/ a 50% chance of winning the Sweeping Advance. The results are
a) They stay in combat. Annoy, Annoy, Annoy.
b) They break combat. One Marine less, the Marines run towards your board edge (useful if they are on a back objective defending).
c) Your drones die. Well what did you expect, you assaulted with Tau?
The Drones can also sacrifice themselves. Didn't wipe those assault termies? Crisis team in danger of assault? Sternguard with low AP bolters bearing down on your squad? Drones to the rescue! Your Drones can form a mighty wall of cover save or assault buff, protecting your more important units from danger, conserving your overall firepower, and making your opponent waste military muscle.
So in conclusion, are drones more killy then FWs? No.
But they avoid putting all your eggs in one basket, and give you significantly more flexibility.
