Well for starters he kept the UK armed forces out of the Syrian conflict when the PM was hell bent on getting involved. I for one will always be grateful to him for doing that. He also challenged Murdoch's buyout of the remainder of BSkyB shares, which is probably why the sheeple think he's so useless.
Perhaps, just perhaps, if we had got involved in Syria the humanitarian disaster that's ongoing there may be less so or even avoided? And maybe a few 1000 less migrants wouldn't be waiting to head here over in Calais....
I think Cameron's reasons for wanting to get involved in Syria stack up a lot better than Blairs (if we ever get to find out what really went on there!) reasons for Iraq, one was principled- rightly or wrongly we'll never know - the other was sucking up to that "wonderful" US Presudent Bush.