I remember from my military NBC (nuclear, biological, chemical) training that sometimes, an enemy can use combinations of chemical agents. For example, they could use a nausea inducing agent to cause someone to take off their protective mask (puking in your mask is no fun) and then hit them with nerve gas.

A lot of my training predates chemical suits, so it's likely dated by events. Still, the idea of hitting someone with multiple threats is to probe for a weakness. Blistering agents like mustard gas are hell on any exposed skin, not just on the lungs. If you catch someone with mustard gas, either alone or in combination with something else, before they can don their chemical suits, then they quite likely would be in much more danger than being exposed to a single agent at a time.