Airlines do NOT have generally reserve aircraft. It would be extremely bad business to sit multimillion dollar planes on the ground just waiting for some technical mishap. Finding a replacement aircraft in the event of looming cancellation depends on how the schedules and rotation of other planes work in relation to the problematic flight... (just had one cancellation last week due to lack of equipment and staff). But nautrally for maintenances the plane is replaced, but that is of course because the same plane never flies only the single route (what we have here due to simplicity).
And also, Weather cancellations are not that rare... Yes, modern airports have the equipment but let's remember for example London a month back where half of flights were cancelled during a few days due to wx (which was a bit ridiculous..but still).
And we are not building a monthly schedule view either, that would be horrendous to manage for the players if you think of it a while...
Here's a nice link for US "live" cancellation data http://uk.flightaware.com/live/cancelled/yesterday
Take SkyWest for example, their website says they operate 1600+ flights a day (http://www.skywest.com/about/gen_info.php
) and yesterday they cancelled 46 flights, making the 2.8% of all flights cancelled. Not that far off in AWS! Especially since the cancellation % is recommended to be kept below 2% to avoid CI problems.
Colgan Air, one reference quotes "more than 350 daily flights" (which I suspect is a bit low, but still..), and flightaware lists 24 cancellations. So that would be 6.8% cancellation rate already.. Are they losing their Certificate for this?