There are three "base" blocks to a venom army, all costing 125pts.
5 wyches with haywire grenades (in a venom with an additional cannon)
5 Kabalites with Blaster
5 Wracks with Liquifier
All of these are effectivly suicide units
I believe we should extend that to include
5 Incubi, naked, 175pts.
"But!", I hear you exclaim, "thats 50 more points!"
But those 50pts get you
3+ armour save
5 power weapons would cost any of the above squads 75pts alone, and they would be AP3.
Now, its more AI, which is the last thing the DEldar need, but its also more AP2, which we frequently lack.
I'd think nothing of sacrifing a 50pt wych unit to kill a 100pt dreadnought, so why question sacrificing a 110pt incubi unit to maul a 220pt space marine squad? Point for point, they beat terminators, although thats a poor matchup.
Obviously, they arent troops, which is unfortunate, but even if they cant claim an objective, they can contest the pants off it.