Prior to expansion books, the difference between the two at low levels(at least) is fractions of a point of damage per round across some of the range of possible target acs.
Not really.
Level 1 ranger. 18 dex. Longbow. Weapon Focus.
If we assume a 50% base chance to hit, you get:
Twin strike: DPR 9.81875
Careful Strike: DPR 6.35
So, in the range where the game is designed to generally occur, Twin strike does more than 50% more damage.
"But wait," you say. "Twin Strike is supposed to be better when it's relatively easy to hit. The time you want to use careful strike is when you are have trouble hitting.
So let's compare the two:
Attack number you need in order to hit Twin Strike DPR Careful Attack DPR
13 8.13375 5.35
14 7.265 4.85
15 6.37875 4.35
16 5.475 3.85
17 4.55375 3.35
18 3.615 2.85
19 2.65875 2.35
20 1.685 1.85
21 0.99125 1.35
22 0.99125 0.85
Looking at these numbers, you will notice that careful attack is more than 1 point of DPR behind twin strike until you would ordinarily need an 18 to hit, and only pulls ahead if you would ordinarily need a 20 or 21 to hit--and even then, is only ahead by a fraction of a point of damage per round.
So, careful attack is straight-up worse than twin strike in nearly every situation--even in the situations that it is designed to make you think it might be a good idea. The very few situations where it is better:
A. Don't come up very often (how often do you only hit on a 20 with twin strike)?
B. It is only ahead by that fraction of a point of DPR that you mentioned
C. You should probably be doing something other than attacking anyway. It's time to think about running away or using aid other or something like that.
The idea that system mastery was designed out of even the initial core books is abject nonsense.
Also note that while the Monte Cook article does mention system mastery, it does not say that it was a conscious design decision in 3.x to encourage it; rather he says, "mostly, we just made sure we didn't design it away."
So we're told that system mastery was a conscious 3rd edition design philosophy--as though the designers had gone out of their way to make sure that the game was hard to understand and that was one of their primary goals when designing the system. And we're told that 4th edition designed away system mastery. I suppose one could get more disingenous, but it would be difficult.