hanifar,
You say that "a scrambled aircraft would expect to be on scene before the SAR helicopter".
That's not always the case. Callout time for a UK SAR helo is 15 minutes to airborne other than during night hours. Towards the end the Nimrod MR2 was holding a 2 hour standby for SAR missions, with the crew at home on pagers. Yes, ex MR2 guys will tell you they would expect to do it in less than 2 hours, but almost certainly from home it would take at least one hour. Transit time from Boulmer to on scene (I believe it was approx. 100 miles off the Northumberland coast in this tragic case) would probably put a SAR helo on scene as the MPA was getting airborne.
Yes, MPA are faster, and in general your point has some validity, but as I have just pointed out, you also have to take into account callout and transit times. If the incident was off Cornwall for example and your MPA is in Scotland and your helo is from Culdrose...