This is a bit confusing... What you mean by 'full node' here? Is there any alternative 'light' client for SpeadCoin? Isn't any wallet a full node? I don't get what 'leaches' mean here - are those wallets which are not yet synch-ed?
The difference between a wallet and a full node is that the full node allows incoming connections and that it has port 41678 open.
A normal wallet only initiates connections to the outside world (to full nodes ONLY), it never accepts incoming ones (exactly because its port 41678 is closed)