While I initially found it a bit frustrating when people turned down challenges because they were up to their limit in terms of games, it now strikes me as sensible. I play most of my chess on another website, where members are allowed to play in as many tournaments as they wish. This means that there are some players running 750-1000 boards at any one time. One in particular, who objectively is a pretty useful player, with a true rating in the 2200-2500 range, goes off the boil from time to time, and suddenly he's being timed out on hundreds of games, which makes his rating plummet very quickly to 500 or thereabouts. This means:
a) If you do beat him in the midst of this havoc, you find that your efforts have come to nought, because by then his rating is so far down that you get no points for the win.
b) He then comes back and enters matches or competitions that are designed for weaker players, to give them the prospect of winning something, and proceeds to wipe them out, which is mean.
At least here the number of matches affected by people like him are relatively few, and the timeouts have less effect on his ratings, and those of us who beat him.