Let me check to make sure I'm understanding this right:
Both clients do approximately the same in terms of mining
The modified client significantly reduces the load on the server
A reduced load on the server allows more users to join the pool
More users in the pool allows the pool as a whole to find blocks more quickly
I'm not seeing the disadvantage here....
(I simply cannot stay outside)
Yes, less load on server for +/- same total pool hashrate. But you (as a miner) are not motivated to have huge hashrate on server, you're motivated to have high OWN hashrate. Higher pool hash rate does not make your payouts higher (only more steady). So with long ask rate to server, you're cutting your effective hashrate and losing some valid shares because you work on stale jobs.