If Agassi is a 3/4 (which JY says it is), then it's a pretty conservative SW. Meaning the palm of his hand is still on the bevel 3 rather than under the bar. Blake and Safin is said to have similar setups, and as you can surmise, they all have similar approaches to the game.
And it makes sense if you think about their games. The grip is not just about the spin you impart, but the nature of your game you want to play and your own eye/hand limitations. This grip, like the Eastern grip, is made for taking the ball early. However, if you let the ball bounce to shoulder high, then it becomes more difficult to smack on the ball and impart enough topspin to keep it down. If you're tall like Safin, this isn't a big deal. Otherwise, you'll have to leave your feet in order to hit the ball in the sweet zone, or play very conservatively. In other words, if you're using an Agassi grip, you better have good eye-hand to consistently take the ball early.
A guy like Roddick who uses a true 4/4 SW, taking the ball on the rise (at pro level) just isn't his game.