[AppyPappy]

He can express his beliefs as much as he wants outside that building and away from his official function.

And the government could just as easily remove any speech against the government in a government building by claiming that speech was allowed elsewhere. Government officials could legislate without any criticism by hiding in the building.

because any government official could try to trump out legal challenges to establishment by saying "I was just engaging in free speech".

That's called "free speech". As long as they don't force others to comply, it's the cost of freedom.

[Chancellor Palpatine]

So in other words, the establishment clause is unimportant to you, and you want one particular sect of religion touted by those in a majority position in your government, using government facilities as part of their "free speech rights".