To really justify including them you will need to consider your enemy's missile fire capabilities. Fighting Wood Elves, Tomb Kings, Empire or even High Elves then forget them - especially squig herds.
Both herds and hoppers are very vulnerable to bow fire, even ordinary, normal, bog standard bows - even skink blowpipes come to think of it. With a T3 W1 and no armour save you're asking to be turned into a pin cushion.
Hoppers are slightly better off because they're skirmishers but this doesn't help them much when it come to fighting.
I think these ugly critters only really bring novelty value to the party, like a person who makes a huge effort for a fancy dress party when everyone else is just there for the free drink.
Herds are better value than hoppers if you want to use some in your army. Hoppers might have the better tactical advantages though, being able to close with the enemy a lot faster (an average of 10.5 inches per move - 3D6)
Neither will stick around in a fight and neither brings any missile weapons to the party. They will quickly get cut down with their low initiative if they don't charge into combat. If they do charge in, there is some potential for kills (squig str 5) but it's unlikely they will win any significant combats.
Note: herds have one powerful, not so secret attribute. If they lose their handlers they go wild and all units within 2D6 (average 7") take D6 str 5 hits. This makes them a bit more attractive if you believe your enemy's forces will be bunched up and you're confident you can get them across the battle field withouth them getting shot to pieces.
Notes: to deliver your squigs safely across the table maybe try popping them behind a unit of night goblins who are on their doom march with their fanatics payload.
The fanatic unit will probably attract all the missiles (like flies to ....). Particulary if you're fighting Brets or stunties who both hate the randomness, chaos and damage to their armour saves that fanatics so enjoy :)
