There's something to be said for undefined SC callout. With a defined callout, a lot of people will tend to just shoot for being there at 2.5 hours, even if they could get in sooner. With undefined, you don't feel like you're "helping" the company by coming in "early', in those cases where it might matter (duty day limit, etc).

I don't know statistically how many reserve callouts occur inside 2.5 hours from departure... I'd think most sick calls occur well outside of that since if you make a habit of creating operational disruption with last-second sick calls, you might get scrutiny. I'd think the real short requirements are more things like duty limits caused by mx, wx, etc.

Also... hot reserve at a big hub has utility to the company, especially at peak times or during IROPS. Where it comes in real handy is outstation disruptions... you can DHD a replacement crew on an outbound flight without delaying it. I'd be OK with *limited* hot reserve if it came with a premium of some sort (more pay, day off credit, whatever)... can't speak for the company of course.