Parasited marines usually become reckless to not be a thread to the platoon. Up to throwing away all non standard weapons and charging in with pistol and melee only.
Now, if a parasited marine would spawn cysts, they'd still be usefull in their suicide rush.
Sure, they might overthrow the aComs plotted cyst structure, but in the end it strengthens the presence of the aliens in that room and actualy helps to underline the alien nature.
Also, a killed skulk should be able to spawn a cyst as well at a certain tech level. Does not work with xenocide, but killed by anything else besides a deathpit should let them spawn.
maybe even make them their own kind of cyst (pustule?), so they are easily recycled, without giving resources to the team to keep them in line with their "effectively for free" nature.