It’s an inyan of derech eretz, a quality that is sadly lacking in many shuls… including the one my husband davens at but where I refuse to daven any more. I agree with both sides (CTLawyer & Rebbetzin) Makom Kavuah should be adhered to, especially for someone who is there almost every Tefillah when shul opens up. It is disrespectful if someone gets up to leave for a few moments & comes back to see her seat occupied by someone else or her siddur gone.
Re Kiddushim: what I can’t stand is people double-dipping into salads, using their used fork to spear food & kids picking up pieces of kugel, cake etc with their hands, often sniffing them and putting them back! What kind of homes do they come from?? But the good thing is, it keeps me on a diet! No way would I eat from those plates.