The Giants would be smarter to deal their 6th starter for a position player. Resting your #1, #2, #3 pitchers an extra day just means they pitch less and have that much less of an impact on the team's success.
Realistically, they made the bonehead move of the decade in signing Zito to a exorbitant deal, so they sealed their fate for the next few years with that blunder. When the baseball gods see a general managing snafu like that one, they press the "Smite" button and curse the team to a few seasons of 90-100 losses. It's that simple. I will always love the Giants, but they seem to be taking three steps back every time they make two steps forward. Hey, while we're at it, let's re-sign a 41 year-old light-hitting shortstop who had a 62 OPS+ last year to a mega-deal so that we can't afford other free agents that could help us.
And Sabean's assessment of "Zito going through with his struggles" is somewhat interesting. I expect Zito to break out of these struggles by 2019, when he's in his 40s. Note to those in denial: Zito has a losing record over the past 5 combined seasons. His struggle isn't temporary.