I also think that if it's a mechanical issue, most airlines will rebook you in anything reasonable (so if its AC in 2 days or AA today, they'll put you on AA, but they won't do it to save you an hour unless you have a really valid reason).

As for weather issues, I don't know. A friend of mine flies regularly to the US with the big 3, and they never want to book AC on weather cancellations.

Nevertheless, when .... hits the fan in the easter seaboard, it's not like youre going anywhere anyways, regardless of which airline you're flying. And say you're in FLL going to YUL with a weather cancellation due to weather in the YUL area, routing on say UA through whatever NYC might not be a wise option. You'll grt closer, and may just end up stranded with the connecting flight to YUL cancelled due to weather as well.