How does that help
The phone lags terribly for a couple seconds when you turn on wifi, like when using the power control widget scrolling to another screen is very choppy. Having it not turn off isn't the solution
Actually, it IS the solution. The phone has some issue when connecting and obtaining an IP address. If you install a system profiler, you will see that when WiFi is powering up, and when the phone requests an IP from the router, CPU usage goes to 100% on BOTH cores and stay there until the connection is made, then everything settles down.
The reason this "fix" works, is because when your screen goes to sleep, the default behavior is to turn off WiFi, which means when you wake it up, you get that CPU spike again. Setting it to never sleep, means you will only get that spike when the initial connection is made (when you first turn on wifi).
This problem was driving me nuts, but i haven't had an issue since disabling wifi sleep.