I've also found it helpful in dealing with/preventing shin splints when done for high reps. But, yep, my initial reason for including them was seeing a Dorian Yates article wayyyyyyy back when, where he pointed out that it was an often-neglected muscle that can add "dimension" to the lower leg.
The trick to the seated one-leg version is to not have your leg at 90 degrees (like you're doing a seated calf raise). You get a much better range of motion in the ankle when your foot is farther out in front and the knee angle is, like, closer to 135 degrees or so.
Also, I generally prefer dumbbells to plates. It's easier to sit the dumbbell flat across the whole instep, and more comfortable (relatively) when using heavier weights.