I'd say the example of the GPU farm I posted above perfectly illustrates a leech:

it squats almost half of the open ports on my full node, which means those ports aren't available for other peers anymore,
plus this farm doesn't even have the decency to make its IP accessible as a full node, since its port 41678 is closed.

That has "leech" written all over it.

No, of course a newcomer that needs to sync is not a leech. Baby needs milk, we all get that.