funny that you say that. i just noticed that twice today in a cafe. why frankfurt of all towns? munich makes sense for this to develop because of their biergarten culture with long benches and long tables
Not too uncommon in Bavaria and Austria, if the restaurant (or especially beergarden) is quite full - the waiters will even sometimes just place you there or give you the option to leave, or wait. I wouldn't say I've seen it if there are free tables though - but I've also been to regions where this would never ever happen, where your table is yours and yours alone.
I'd say talking to the people at your table is purely optional, sometimes it works, sometimes it doesn't.