Is there a functional reason for the offset?

Installation-wise, due to the possibility of slight bowing of some 12x24 tiles along their length, a stacked (no offset) pattern will minimize any lippage due to the bowing. Example, if the tile is bowed a bit along its length "(", the highest part of the bow will be at midlength, 12" from the end of a 24" tile. If you did a 50% offset, then the highest part of the bow in one row would be next to the lowest part of the tiles in the adjacent rows, their ends.

That's why for long 24" tile like yours, the maximum offset recommended is a 1/3rd offset.

A straight stacked pattern would minimize the issue.

An easy way to check for bowing is to take two tiles and put them together, glazed face to glazed face. Any bow in the tiles will indicate itself like this: )(

Other than that, patterns are a personal choice. They lend to the style of the room, for example. Some patterns can add detail for a period look, others can be more linear for a modern look.

Thanks for the reply - so now the question is whether style is worth an additional $1,200! :)

Did you get other bids? An extra $1,200 to install the tiles in an offset pattern sounds insane.