Drop leg holsters don't give you any kind of advantage in speed of draw. They do give you a disadvantage when it comes to weapon retention. Carried on the thigh, you're prevented from clamping your elbow down to secure your weapon as you're able to when worn on the hip. As BravoSierra2012 pointed out, drop legs also cut down on your mobility.
Modern drop leg rigs, for weapons, magazines, etc. were designed to solve a specific problem, that of clearing body armor. Why would you wear an answer to a nonexistent problem? I've got a good amount of time carrying an M9 in a drop leg rig. It catches on doors, deprives me the use of a cargo pocket, and just isn't comfortable. But, it does place the weapon below the level of my vest. The thing it is designed to do it does well.
For what it's worth.