None, I don't think lings really need more power-ups from absorbing people. I don't think think the entertainment value of potentially having a host of your victims moan at you is high enough. I think we all remember those wizard rounds of old days, where you didn't even need to actually understand them to get a giggle out of all that muffled speech and spinning chairs.
For cultists or constructs one could make them witch in and out of the host (e.g. as ghost/shade/wraith) for actual gameplay benefits such as spying on people or 2spooky ambushes where a cultist has multiple spirits pop out of him.