Besides the fact that it really can't take the power, an instrument cable is shielded. Speaker cable is more like lamp cord, it's two equal conductors with no shield.
Part of the problem with using instrument cable is the shield wrapped around the single core conductor. That conductor in the middle is a little too small to take the power, so it will heat up and cause resistance to the signal. Since it is wrapped in the shield, the overheated conductor can't shed the heat. This could cause failure, which amplifiers hate.
If you were going to GC to pick up a 3 foot instrument cable, it would make more sense to just pick up a 3 foot speaker cable and do it right.