Many Jiu-jitsu schools discourage, and even disallow, the use of leglocks while sparring. They argue that leglocks are a ‘cheap’ technique, and/or are unsafe, and/or prevent you from developing a good guard passing game.
I respectfully disagree.
The ‘cheap’ label gets used for new and unfamiliar techniques. There was a time when kicking someone was considered cheap and dirty – now with a more educated public it’s considered a cool way to knock someone out (thanks Bruce Lee).
Are leglocks dangerous? Well, it depends. All submission holds are dangerous if applied in a ballistic manner and without control, regardless of whether they target the knee, ankle, shoulder, elbow, wrist, etc. Ankle locks, compression locks and kneebars are fairly safe, if applied with control.
Leglocks that TWIST the knee and ankle, such as the heel hook or toe hold, are definitely dangerous. These twisting leglocks have a very narrow safety margin: the onset of pain is often indistinguishable from onset of damage to the joint. Be very careful if you use these techniques – always being ready to release them if your opponent starts flailing or refuses to tap.
Now I agree that novices who concentrate on leglocks a lot tend not to be great guard passers. However most leglock specialists eventually figure out that they need to make their game well rounded, and then work on developing their guard passes. They get to the same place just via a different route. After a certain point leglocks and guard passing actually complement each other: for example you might fake an ankle lock and then go for the guard pass.
Leglocks are becoming much more popular in competition. To learn the offensive and defensive aspects of a technique you NEED to include it in your sparring. If the knowledge stays theoretical (i.e. you are shown the counters but never get to use them on the mat) you will never hone the sensitivity and awareness to defend leglocks at a high level.
Without the threat of leglocks many beginners develop unrealistic guardwork, leaving their legs WAY too exposed. Without the threat of leglocks how will you ever learn to defend them properly?