I've got an HDD that I sometimes run a full node on, but since HDD's are slow, I'm thinking it might slow down the network rather than speed it up.
Does it make much of a difference past the initial first sync?
In any case, internet connection bandwidth and access is far more impactful to the network.
Using a spinning disk, it's just you who will feel the pain during your initial full sync. Once you have got the blockchain on your spinning disk however, there is really no issue keeping it synced up, so long as you keep it running.