Certainly the Ten Commandments could be interpreted as a historical document - not nearly as important in the history of the United States as the Magna Carta, but being one of the most famous law codes in history, it probably has a place in a museum exhibit of ancient law codes.
The idea that the Ten Commandments promote morality, or that they are compatible with American law, however, is absurd. It's difficult to discuss individual commandments given that nobody can seem to agree on exactly how they should be divided, but assuming the Protestant divisions, the first four commands are to do with the worship of the Hebrew god. They have no place in modern law, and even among ancient law codes, seem particularly benighted. Two more are good ideas, but should not be the subject of legislation (honouring parents, not committing adultery). The prohibition on "coveting" is absolutely antithetical to the American ideal. Finally, the three important commands - those prohibiting murder, theft and perjury can probably be found in every known law code since the beginning of time.
I am baffled as to why anyone would want to display them, especially given that even Christians don't believe they have to follow them. Displaying them as anything more than a historical curiosity would seem to me a violation of church/state separation.