I know a family that did this (they have over a dozen kids). It worked very well for them (after they expanded their dining room). Some tables look better stuck together than others of course. Simple farm-style trestle tables can work pretty well.

We do it all the time. (It's also what restaurants do to accommodate varying size groups.)

Just throw a 12ft table cloth over them and nobody will ever know the difference.

That said, 12ft of table at a 12ft bench (which I assume, as a built-in, is up against a wall) means the person in the middle of the bench has 6ft of family on either side of them that they have to slide past to get in and out. Kind of a pain.

It can be more practical to have two 5ft tables. The 2ft gap in the middle makes getting in and out much easier.

Be sure the legs of the tables are built so you don’t lose seating because the new middle part of the combined tables are impossible to position the people legs.