There are several single and double leg takedowns and IMO they are practical in sport and many actual combat situations. The question is what you do after the opponent is on the ground (no matter how he got there). Leg locks seem hard to achieve without having excellent skills, and even then, I have seen many reversals on MMA when one slips out or rolls over. This may be OK in sport, but in actual combat, seems a little risky. For me, a quick stomp, knee drop, strike or twist on the ankle or knee joint (groin, or anywhere else) is more direct and less risky than getting tied up with the guy trying to hurt you. I would go for the quickest, simplest thing that is available and build off of that.
On the other hand, if I was very proficient in such maneuvers and found myself in good position with a leg in my arms, I might go for a leg lock - don't look a gift horse in the mouth - take what you're given. But realistically, I would think such ideal situations would be quite rare.