Not a stupid question, its almost impossible to answer this one, with any confidence, as what ultimately happens depends on many factors. Rubbing two materials together may transfer electrons from one material to the other charging (or de-charging, that is removing electrons, really makes no difference, so I will call this one charging too) from one material to the other. if that happens the plastic case might be "charged" with an excess (or shortage) of electrons, compared to say earth (which we will assume has a "neutral" amount of electrons). so if there is a sudden potential discharge path to earth, that happens when the air between the charged case and the potential discharge path becomes ionised, the charge might be suddenly discharged to earth, and "sparks will fly". it depends on which path the discharge current takes if any damage will occur.
Probably not much will happen, as the plastic case is a good isolator, and the charge will be on the outside of the case, as electrons repel each other, so no electrons will flow -through- the PI.
I hope this answers your question.
I would not worry to much about this, the plastic case does exactly what it is designed to do, and that is to protect the inside, by being a very bad conductor. Think of it like this, if you walk over a carpet and get statically charged, then touch a bare PCB (PI) then the spark current may flow through transistors on the PCB, and potentially may damage the PI, but if you instead touch the plastic case, the very high resistance of the plastic will limit the current flow, and electrons will discharge slowly.