[Imp]

You answered your own question pretty much on why it’s not worth including.
Gas attack and then attack, if your evenlightly wounded your dead because no medieval hence you would not do it unless it was against someone with no defence against it.
Could conceivably be used to make no go areas so just place swamp terrain to show contaminated area. Slow movement and poor defensive rating. Do as a one off if you like but that’s what it would be.