The old adage that you can never have too much pitching usually turns out to be true. You might have 4 good SPs, which is unusual to have, yet that means you also have a bad one...and then a couple guys get hurt, and...The same pitcher who is fine as your #5 SP and is "serviceable" as your #4 is also way stretched as your #3, and if he's ever suddenly your #2 then congratulations: You're the Royals. Depth has a way of being tested.
However, like all adages (and many subtractages), it's a principle but not an absolute truth. There can be downsides to going overboard in accumulating "too much pitching".
What stands out first to me is that the big "jump" in quality goes between #2 and #3. Essentially, the A's currently have two legitimate "front of the rotation guys" (Anderson, Parker) and then several pitchers (Colon, Griffin, Milone, Straily, Blackley) who slot in somewhere in the back of a rotation -- call it "#4-ish".
That's good depth -- and at the same time it would be an upgrade to bring in, say, a true #3 SP because that slot currently gets filled by someone who might be worthy, or might not.
However, bringing in another SP would also use up resources that could be allocated elsewhere, be it towards a SS or an upgrade at C. Plus, maybe you bring in a "true #3" and what do you have? By nature, #3 SPs are talented but erratic -- talented enough to be rated as high as #3 and either inconsistent, untested, or risky (health? age?) enough to be rated as low as #3. There's no guarantee that pitcher is going to be better than Straily or Griffin, or more consistent than either of the "ohn brothers" (Colon, Milone).
Which brings me to another key point: Bringing in another SP means "stashing" another young pitcher. Already, if everyone is healthy at the end of spring training, someone is going to be "odd man out" to start the season. Unless it's Colon going to the bullpen, it's Griffin or likely Straily.
OK, not the end of the world: Straily blazed through the minors last season, struggled at times in his big league cup of coffee, and could stand a little more AAA seasoning awaiting the first injury. But if you start stashing the likes of Straily and Griffin, now you're starting to do more than just ensure a parade of #4 SPs that can arrive faster than they can fall. You're also stunting development as you make "major league ready" pitchers bide their time in AAA. You're using their service time for "thumb twiddling," not for "snapping off breaking balls. You're taking the "X number of pitches their arm has in it before it goes kablooie" and you're spending a chunk of them on "making sure to get Hasbeen McRetread and Whiffy McNeverwas out".
Do I want my AAA rotation to be full of "young big league ready #4 SPs"? Well, sort of. I do believe that if you're the "first guy to get called up from AAA" you're going to throw a lot of major league innings. At the same time, I don't want legitimate talent like Straily and Griffin, Milone and even Blackley, to atrophy on a minor league mound or a major league bullpen.
I did want the A's to re-sign McCarthy, not just because I love the guy but also because he's a clear upgrade (when healthy, of course) over Milone, Griffin, Straily, and Blackley. I didn't really want the A's to re-sign Colon precisely because he isn't a clear upgrade -- he's more like "another one". Of course, McCarthy turned out to cost about $15M over 2 years to sign, and the A's now have some payroll flexibility left for having let him move on. Colon cost about $3M (plus possible bonuses) to sign, and the A's now have some added depth going forward for having brought him back.
The A's have said, and I agree, that you don't build a 5-man rotation, but rather you build a rotation for a 162-game season. That usually winds up being about an 8-man rotation -- but when and how much you need starters 6, 7, or 8, and maybe 9, is a big unknown.
So with the limited resources a team like the A's will have to try to bring as much talent, and depth to the season, in as many areas as possible, how much pitching is enough, and when is it "too much"? And have the A's gotten it right for 2013?