My understanding from the weapons expert O'Reilly had on last night, was that degraded means the stuff is no longer a highly volatile liquid, but has turned into a sort of goo. It doesn't flow well, so the artillery rounds wouldn't work properly. Seems to me the terrorists haven't exactly needed artillery tubes to make artillery rounds do their bidding up to now.
That is only for the mustard gas shell and not the Sarin shell. However, there is a thread going around showing the UN says this mustard gas is in quite good shape after 12 years of storage.