To go back to the OP's question for a moment. Simple. Yes it is a good use of the military. It is part of the contract. What used to be called 'Aid to the civil power' is one of the reasons for having armed forces. ...
If you don't think the armed forces are there for the national interest, then you may need to review your terms of service.
MACA/C/P are indeed valid roles for the military, but I do sometimes wonder whether or not that rather than being used as a last resort, the military has become the easy answer to every day rather than difficult questions. As soon as an issue that looks like it might be a bit tricky comes looming over the horizon, the Bat Phone rings and that's the military mobilized to help a spate of cats stuck up trees, de-icing pavements and filling in the gaps in Olympic car parking because it's too difficult / expensive to do it any other way, and as the military are there and paid for, we may as well use them.
I guess what I am suggesting is that whilst the military are professionals and will do what we are told by our Lords and Masters, it does grate more than a little to find out military roles being whittled away by years of underfunding, downsizing and capitalising on various 'peace dividends', only to become the Government's Handy Man. In some ways, it almost feels like we have re-instated the old General Duties trade and enrolled the entire military into it. And as UK plc sells off more of its critical infrastructure and enabling functions, I can only see the military being used more and more as the private enterprises that bought said infrastructure / functions find their wish to make a profit clashing with the Government's need to keep the country running.
And it's a rare occassion, but I do actually find myself agreeing with pr00ne on the Government's recent performance. Shame there is no ability to re-course them for poor performance.