Pre 0.21, or whatever it was when revert flight was added, I would land my Kerbals and would notice that they had very little fuel left, I would realise that my kerbals may be there for a while, so that is why I always send a mission of 2 or more Kerbals (Because I believe it is inkerbalmane to strand a Kerbal for long periods of time on his own). But I know that I will eventually (Hopefully) send a rescue mission to go and get them back. I have so far only sent kerbals to the Mun, because I believe you need to first land a probe, etc. But it is also because that if I send them any further (excluding Minmus) I may never be able to send a rescue mission to them. But this was all before I could revert, so usually if a mission goes wrong I revert now, so they are not dead!