At the moment I kick with both legs when sparring however I find that I think about the stance and then which leg it is best to execute the best technique with. I think it kind of depends on what sparring rules you are following.
TKD seems to mostly be continuous sparring where massive combinations can be used and its important to keep drilling in with kicks. Karate is mostly point stop so you need to place moves and pull them back really quickly so sometimes low (leg) kicks can be used as a distraction and then you can hit your opponent with a quick scoring punch.
When I was doing the TKD style sparring I found that my left leg was absolute trash for kicking from and was mainly used as a pivot for spin kicks on my right leg. I also found that when throwing a lot of unexpected twist kicks, hook kicks crescent kicks etc then I could hit the angles I wanted with just my right leg.
Now that I am mainly practicing
karate the rules have changed. I can't make use of kicking from just my right leg because I will get my left leg swept so I continually change which leg I lead with. I tend to find that I can kick better off my left leg when I am leading with my left leg because I can then use the momentum to drop to my knee and do a rising reverse punch and spring out pulling back so I get the point. My right leg is still fine leading with the left or right but I still have to watch out for being swept.
I think what I am trying to say in short is that its important to think of how you want to use your kicks depending on the situation. Maybe I'm wrong though
