So how does it help the pool at all, or "contribute to the pool" as you said?
Since the fee can only be spent by Eligius, it is in effect a donation that the pool also accepts in lieu a transaction fee.
So the thing that makes it a transaction fee is the destination address. So without OP_NOP, the destination address would still allow Eligius to pick up that donation, but the only difference is that the transaction might be included in a non-Eligius block.
So they're paying as a thank you for Eligius mining it, but the only reason no other miner mined the transaction is that they changed their transaction to prevent them from doing so.
This is really pointless.