Where do we get the guns?  Even if you can find them, it brings enormous new risks.  Without a powerful alley willing to supply & help us stand up to the tyrant, we, and by extension the rest of the world, would be fcuked.  Furthermore, just remaining well armed when things are going smoothly (and despite our problems, things are going relatively smoothly now) is a deterrent to the government overstepping its bounds. 

This makes sense to me --> to avoid tyranny.  Should you happen to be ruled by a tyranny somehow the constitution isn't much of a help I think.