Sorry to hear that. All the more reason to board early, I guess. However, I do think there is a point to starting an argument or, at least, making an issue of it. It has yet to happen to me, but if it does, I will tell the FA, "I'm not sharing my seat with anyone. If I can't have a full seat to myself, then this is an IDB situation. Please call the purser, because this boarding pass says I'm entitled to a seat and, evidently, there isn't one for me." And I'll hold firm.