We do however know that there are a third as many of them as imps. I think that numbers are valuable, overkill and fronts and all that... Granted, equipment costs do increase the value of a few expensive units compared to many cheap ones, but with judicious use of equipment, such as having cheap archers with just imp+caster, then you can still benefit from numbers. If you then get to compare them to kitchen-sink generalists then you can double your numbers for a third of your budget. Granted, this would only be of value if the caster-troops are worth anything, but, well, it is part of our basic package and twice as expensive as a really sharp dagger that can attack, well, a lot more than once every several minutes. I am trusting that we weren't given a complete and total dud here and hoping that the initial salvo will be enough to damage morale and that I have put in enough forces to hold the line while the archers prepare for another salvo. If a battle lasts half an hour, and the archers average a kill once every ten minutes, then that is well worth it. And even then, only a fifth of my budget is dependent upon the things.
And while Blackscales might be possible to fix with a revision, that revision could fail, and their armour and close-combat weapons are subject to the same issue. And certainly they could also be fantastic super-successes, but that is only a chance, and it leaves us with no variation in our forces...
Ultimately though, it comes down to a question of whether or not our starting options are inherently weak. If flame casters keep going to their maximum range and will burn through a hundred blackscales if they all stand in a line, then they are probably worth it. If the projectile travels as fast as the average toddler and inflicts a mild sunburn, then no, they aren't good for much. I don't have a way to discount either of those based upon the description. That is a bit of a toss-up too, which is why I have a mixed force.