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.
Use enough so your enemies slow down and send probes in first, and you have gained valuable time. Detection can be used by more than just the Speed Demolition races. Certainly they are the ones that use them as scanners, but other races can use mines as early warning system. A forward mine belt, suddenly getting cleared, gives advance warning of a major fleet movement. No great speed required.