Nothing prevents conventions from being called other times.

To me, this is like inviting people to submit their versions of how the Constitution should read.  Calling for such a convention any other time would look like an obvious political move.  This provision allows people to hide behind the Constitution itself.

Without research as yet to the contrary, I'd favor amending to eliminate this very provision; make people work to prove their schemes from start to finish.

A Convention is the worst thing we could do to the Constitution.  If you don't believe me, read the section providing for the 2010 vote on it.