At least one business advice site say elbows on the table will interfere with your neighbor’s ability to eat his/her food:
“Keep your elbows off the table while eating. It can interfere with the person seated next to you. After the meal has been cleared, you may put your elbows on the table in order to lean forward to join in conversation.” (http://entertaining.about.com/cs/etiquette/a/tablemanners.htm)
And another anonymous guy says it is because when your elbows are on the table it makes you hunch over so that there is pressure on your stomach that interferes with digestion.
I also see several sources of the one-sided trestle table tale but no actual documentation.
And then there is this: “Some people think we don’t put elbows on the table because, while most animals don’t have elbows, our close cousins the apes do. We don’t like to think we’re apes.” (http://weekendamerica.publicradio.org/display/web/2008/02/29/manners/)
Most of our modern etiquette is left over from the Victorian period, when upper class people never had enough to do so they invented huge numbers of social rules to differentiate themselves from lesser beings. I suspect many of our “manners” date to that period, although some may have earlier roots. In any case, if you put your elbows on the table you take up more space than someone who doesn’t. That might make them angry and since they already have a knife handy, who knows what mayhem might break out?