In the A320 if " Air Wing Leak" ECAM message occurs during flight the Abnormal Checklist requires that the associated Engine Bleed pb be switched OFF. Why then is the same ECAM Alert a No Dispatch as per MEL?

Boyington: The question should be: why would I want to commence a flight with a known Wing Air Leak! MEL is designed to consider a subsequent failure therefore with one wing isolated another bleed failure would lead to no pneumatics at all.

ECAM and MEL have different purposes. ECAM tells you the way to mange a failure to get the aircraft down it doesn't rectify the failure(unless some reset etc. works). The MEL may permit another flight depending on redundancy in the system and the gravity of another subsequent failure in that system.

A320 Air Wing Leak
 

Leaving aside the in advisability of flying around with a bleed leak(possibly isolated) how would you get the engine started?