This argument fails on several levels.
First, many of the folk wanting to "uphold and defend" are already veterans. They've done the Guard/Reserve/Active route already.
Second, joining the military/guard/reserve is no guarantee of being "on the side of the angels" when it comes to upholding and defending the Constitution. Consider New Orleans. The order was given to confiscate firearms and they just did it. To the best of my knowledge nobody said to this unconstitutional order (violation of 4--probable cause for warrants for search and seizure, 2 naturally, and 5--due process for deprivation of property or just compensation for confiscation "for the public good") "No, that's an unlawful order." They just did it. While maybe the military would mostly be "the good guys" (I really want to believe that since most of the people I know in the military are definitely on that side of things) but can you be sure that you'll be assigned to a unit that is? It's a lot easier to walk away form a "citizen militia" that oversteps the Constitution than from government run military units.
Third, I dispute that the situation is so binary--that one should either join the military or do nothing. Is there really no room for folk to contribute to Admiral Yamamoto's "You cannot invade the mainland United States. There would be a rifle behind every blade of grass" that falls somewhere between being a gunowner and actually joining the military? Or are we limited to a position reminiscent of Goering's "If anyone wants to own weapons let him join the SA or SS where his use of weapons can serve the state?"
Finally, I would like to point out that the Swamp Fox's irregulars, the ancestors of the Army Rangers, was a citizen militia. And that is just one of the many successful militia units in US history.
I agree.