Clearly, certainty would be one advantage for the provinces, but I guess you'd have to balance certainty with the need for flexibility sometimes. For example, you don't want to have a period that is too long, because if there are intervening changes and costs have dramatically increased, you can't reopen the agreement because you've signed a long-term agreement.
It's sort of like getting a mortgage. Do you go long with a fixed rate but be stuck with that rate, or do you go shorter and maybe have the advantage of flexibility? The same type of financial consideration that goes into taking a mortgage out on your house also goes into negotiated agreements. Whether you go short or long, there are advantages and disadvantages depending on what happens in the marketplace, or the so-called legal justice system marketplace.