Bit of a daft question really, due to the word 'necessary'
Clearly some criminals have been apprehended due to police helicopters who would otherwise have got away. The real question is whether my taxes were put to good use burning all that Jet A so the little Toerags could be rapped over the knuckles and let off with a suspended sentence. Personally I think not unless you got my money / car / watch back
As for HEMS, there is far less evidence they actually save lives. In fact none. So necessity depends on what value you put on avoiding an hour's land ambulance ride to hospital with a broken leg......
I suspect at the end of the day it is much like anything else paid for by taxes. The 43.8% of the population who pay no tax want more spent. The 1% who pay 27.5% of all taxes are not so enthusiastic.