I would still buy this set!
But like someone said already why would Lego design such a large set without a purpose.
For the sake of being large? They have never done that.
Also consider this was never meant to be a modular set or be compatible though your never restricted from doing so.
Modulars have a specific design concept that other models do not have. They are meant to be expanded!
All modular sets have this expandability in mind and Lego greatly encourages buying multiple sets to do this.
Even though I would still buy the set without the GB stamp, I would never buy more than one because of the price!
Also anyone who complains that it doesn't have the stackable appeal of the modulars hasn't put on together.
Each level is capable of being removed.
I guess with some serious Mods it could be a expandable modular.
But again....it's a Ghostbuster set!