I've yet to decline an invite on any site regardless who's asking but it's getting to the stage where I just have too many games running at once. I'd probably continue accepting anyway, just drag my heels until I free up some time.
Declining an invitation to play
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I get challenged frequently by players so much lower rated than me that I lose a lot of points if I lose and even a bunch if I draw and get zero should I win.... why would anyone expect me to accept such a challenge ? I decline and give no reason. You shouldnt have to give a reason for declining and in this type of situation the reason should be obvious anyway.
Courtesy to an opponent deserves a truthful reply. Keep in mind, most of the time we don't know who we're playing. Sometimes players are children and adult beginners that do sometimes deserve consideration...
Can't you set your preferences to only accept challenges from players within a certain range? I know you can in online chess, since I played one fellow who timed out, and then wanted to play him again but I was then too highly rated. I got a message saying "Sorry, ______ is accepting challenges from players rated XXXX to XXXX." If you don't want the lower rated player to keep challenging you, then set the lowest rating you'll accept just over his rating.
In principle, I don't think anyone should be obliged to play anyone if they don't feel like it. There might be some exceptions to this, such as a world championship match, or if someone left their work to meet me to play I suppose that we should play at least a few games. On the Internet I really don't see how it could be any kind of obligation.