As of late I have noticed that the main deck F/I has not been pulling it's weight. I have replaced this w/ Gorilla Shaman #2 and I love having two of these guys.
I don't think any metagame factor could make me want to go down to 1 Shaman, other than an uber-scrubbed out metagame. It's such a key component to mana denial, obviously.
However, I've been testing my 4-Color control build without Fire/Ice in the sideboard (in favor of one maindeck, in addition to 2 Shaman), and about half the times I wish I go to grab it and remember it's not there. While the list is just for testing purposes, F/I should definitely stay in the sideboard, even if the metagame in question warrants one maindeck.