generally speaking, you should consider those things when caught:
can i win this fight?
can i escape?
can i at least trade?
if you can win, go fight and win.
if you cant win, go and try to escape.
if you cant escape, go and trade or just die miserably.
you should consider this in the first second you realize you got caught. then, re evaluate again: can i escape? can he escape by himself? can i do something to let him escape?
in MOST situations, it isnt worth to RISK giving doublekill to save a moron. if the adc is in a situation where he gets caught "alone" with his support, he allready made so many grave mistakes that led up to that situation that i totally wouldnt risk anything for him. worst case, you turn to stun someone so he can escape and he thinks you want to fight and turns and gives doublekill while blowing all his summoners making him useless for 5 minutes.