It depends on the guy. The guys where you depend on ratings like power and range? Yea, call them up when they're ready and can help your team immediately. I think pitchers are different because they generally don't begin to lose their "important" ratings until their late 30s, so if you think it's more important to you to delay paying the big money and hold on to him as long as possible, I get holding him back.
I can find instances where I called up guys and probably should have waited the "20 minor league days," and vice versa. It's a case-by-case deal. But yea, I feel there are too many teams who hold back difference-makers and find they just missed a playoff spot, or a bye in the playoffs, or whatever. And eventually they'll find themselves paying a guy in his last year too much money because he's deteriorating.