Quote from heech:
I don't understand the argument that FAS and FAZ will actually go to zero.
It's not like FAS/FAZ point to a specific set of options. These guys are updated and re-hedged every day.
I seriously doubt there's an on-going "decay factor" that can be counted on.
I think "decay" may not be the best word to use around option traders.

If I'm not mistaken, what the OP refers to is the fact that these leveraged ETFs track the daily returns of the index, and hence their multi-day return is path-dependent. In other words, they don't track the index on a multi-day level.