I'd estimate that in 99% of circumstances, it's not worth buying DL RDMs at 2 cents per mile. I've been able to find situations where I can redeem them at that value (often in premium cabin travel), but they are few and far between (and, IME, getting less frequent).

I would buy SkyMiles at 1 cents per mile or less, as that's the floor of their value using Pay With Miles. Beyond that, unless you have a specific redemption in mind where you're just short, I wouldn't bother.