Although it's hobby thing, i think they still help Bitcoin network if they allow incoming connection (with proper router configuration).
Do they help if the RPi is in a constant bottleneck though? There was a time when portable RPi Zero full nodes were being sold around, and these certainly don't meet the requirements for running a full node and incoming connections might only make things worse. The lowest possible version of it that could handle full nodes decently are probably from RPi 3 and up; otherwise, it's all just a struggle for both the RPi owner and those connecting to it