I mean if I can manually quit and have it reopen as I had it, that would be good. As long as there's a manual option there, I don't need to wait for a timeout, nor would I run into the problem of cmd+tabbing to Mail, then going back to Safari only to have to wait for it to resume (ie. I'd leave it completely running).
If you have little memory and large documents loaded, Mail will cause Safari to swap out and you will have to wait anyway. So this suspend-resume thing is just swapping taken to another level (say, being able to sustain a reboot, and not take up both physical and virtual memory when not in use).
Or, maybe, old good "deep sleep" feature of your computer but on a per-application basis.