With Sif, methinks... sorry, I think it's the second time we run into the Marginal Limitation and shoehorn it as Tiring.
I'll explain - the Marginal Limitation was developed in DCH for characters who have a useful power and use it much less than they logically should, for no discernible reason. Sif's dimension travel/astral flight thing is a good example. It looks really useful, no side effect/prerequisite/cost/whatever is ever mentioned, and she almost never does it. Even with the dimmest, most asleep player would see it on their character sheet and goes "hey, maybe I should use that".
So in DCH, Marginal costs Hero Points to activate, which is pretty good at simulating inexplicably under-used powers. The Hero Points are used as a "narrative currency". It comes in three severities (Minor, Serious, Catastrophic) depending on how many Hero Points must be burned for the ability to be bought into play.
I wonder how a M&M Flaw costing scheme for this would work.
(You can also think of it as the character yelling at the writer "you do remember that I have that power, right ?").