[Cheeze]

You don't even necessarily have to mine all 8 sectors around the planet. Most ships tend to follow predictable paths, generally the shortest, to warp points/other planets/etc. You can anticipate their route and place mines in those sectors. For a little while, you might really mess up an opponent's ship travels! Imagine how paranoid he can get when he has to escort every ship with a minesweeper!! Oh the fun in seeing that!

You might even try (if the target lacks sensors) to use cloaked minelayers to drop them over a planet. It's possible in that case your ships won't engage, and he won't be the wiser, until too late, to find out what you have done.