It depends.
If the game is winnable, its a decent kill imbalance, towers aren't too overwhelming then yeah, report them. However, if it is like 20-4 in the first 15 minutes, lost 3+ turrets, and at least one dragon and Rift Herald, they are right not to want to continue.
I got out of a really bad game where we had no chance of winning due to a feeding player. So after this person fed all three lanes, our towers went down quickly and the other team got dragons and our jungler.. well, he forgot smite so that's out of the question. Within 10 minutes it was about 5-2 and in 15 it was 13-4 so it was obviously only getting much worse.
Giving up shouldn't be reportable if its a legitimate gripe (losing so many turrets/objectives and so behind in gold and kills you cannot win the game logically) but if they just are upset they had a bad laning phase or something then it should. Problem is people don't seem to know when they should report and when they shouldn't over it. Riot needs to figure this out.