My friend (whom has one) says that its stats change depending on your sub (whm/blm/smn = mp, mnk/war/thf/drg/nin/sam/rng/brd/bst = def, pld/drk/rdm = both)
This thing seems useless unless you know what can trigger its hidden effect. By looking at it, I'm sure you can somehow make it increase in DEF and MP under the right conditions. I wonder what it is...
There's a tilde ( ~ ) after the numbers for defence and MP. This must be the reason for the name "Variable Cape". The tilde is generally used to symbolize "approximately equal to". So there must be some sort of hidden conditions to make the numbers change.
I don't see any reason for this cape. You're better off just getting a Mist Silk to hold you over until 32. +1 MND is infinitely more useful than +3 MP (even for BLMs, who likely have a WHM sub at that level anyway) and it's an extra DEF to boot.