Better yet, put them right on that pesky wormhole that appears in the middle of your empire. The wormhole will jump around, be prepared to spend some time mining around it. Offensive uses for mines are more than you might think. Those pesky roving wormholes can be handled also by jumping through and mining the other side. Forward defense, or a good offense, you decide for yourself.
Mines are also good for isolating the battle area.
If you are going to bomb a planet into submission then losing bombers to enemy fleets can be a problem. Mine the target system to keep relief missions from entering too quickly. Mines can also be used for an economic offensive. Planted deep in an enemys interior, between starbase worlds and their supporting planets, freight traffic will be crippled until the mines are cleared. Fear is an important part of using mines.