As I said, whilst I don't live close to Heathrow, I live close enough that flying is out of the question - we always drive. For my travels, whenever Heathrow is involved, it is always the departure airport. I appreciate if you're flying from Manchester that isn't necessarily the case.
This isn't the case with competing carriers routing over hubs such as DXB, ATL, or AMS.
Due to the fact that those hubs have sufficient capacity to still accommodate most if not all flights even if there are significant disruptions (especially with DXB and ATL, long haul flights will be prioritized). As Heathrow is "full", if there is any kind of delay, to still handle all flights, aircraft would be taking off well after the 23:00 curfew. Even under "normal" conditions with just a few aircraft taking off late after 23:00, W.London residents get very angry and claim that their right to sleep is being taken away. Thus cancellations follow.
Whilst I don't necessarily agree that all flights should be treated equally (short haul at least), if Manchester were to have say 4 of its 7 flights to Heathrow cancelled, I do agree that the other 3 should be spread equally across the day, and not be grouped together in the evening for example.
What will be interesting to see is that once (it almost certainly is going ahead) HS2 is built (a long way off I know), with 1 change at Old Oak Common and a 15 minute Crossrail ride, you could be at Heathrow 90 minutes after leaving Manchester Airport. Whether or not that means the Heathrow to Manchester flights will be ditched entirely I don't know, but at least if your flight is cancelled, you'd actually have a pretty good chance of still making your onward Heathrow flight (albeit after purchasing an expensive train ticket). Who knows, by then Manchester Airport might have grown by such an extent that the Heathrow shuttle isn't needed anymore anyway.