There's value in flexibility.

Suppose you value points in XX at 1.3, YY at 1.8, and ZZ at 1.9, and you have some CC flexible points that transfer 1:1 to each. CC must be valued at a minimum of 1.9, since they can become ZZ points. But the flexibility to also transfer them to YY adds some value, since there may be times you need YY points; you'd rather hold CC points than ZZ points for that reason. So maybe you then decide that CC is worth 2.0, for that reason.

In other words, flexible points must always worth a little more than the valuation of the highest currency you may transfer them to.

The contradiction here, though, is that if flexible points are always worth a little more than the valuation of the highest currency you may transfer them to, then any transfer will be from a higher value point to a lower value point, and will seem like a "bad deal". That's why you don't transfer speculatively.