The duty is to uphold the Constitution, yes. As of now, eligibility is presumed, and he is considered duly elected. The higher up the ranks you go, the more the brass will take a VERY dim view of a “birther” challenge, because it undermines the chain of command & good order & discipline. (It is a disgrace that we don’t have a clear system to establish eligibility.) The article doesn’t clarify that the named plaintiffs of high rank are retired.
But it does occur to me: Let’s say an officer was charged with an Article 88 violation re: his speech about the President. The officer would then have standing to move to dismiss the charge, on the grounds that he was speaking of someone ineligible to be President.
You bring up a very interesting point.
In a twisted sort of way it would almost be better if one of these soldiers was charged with something because then they would have an expedited route to securing a copy of the vault certificate to use in their defense.
I bet Obama is treading carefully regarding these soldiers because the last thing he wants to do is give them an opportunity to have his birth certificate issued to them. He just wants these case thrown out due to standing.