I lunge for different reasons. When my goofy arab mare has a spaz and looses all focus when on the ground just leading or w/e im doing Ill stick her in the round pen and free lunge to regain her focus. It works well for her.
I never lunge romeo...he doesnt ever need it...for any reason...the only reason I lunge him is to make sure he remembers how to....so maybe once every 3-5 months? I also sometimes lunge him when its to cold or yucky to ride but he needs some exrecise.
I believe lunging can be a way to bond with your horses as well if done right. Im a firm believer in the Join-up technique and I've preformed it with both my horses. Round penning can also be used for respect and punishment.
If Bause losses focus but doesnt throw a fit ill only get her moving for a few minutes to re focus her and its just trotting. This mare is also fat an sorta lazy...but my lord she can pitch one holy fit from hell. In these situations I make her fat Butt Run till she chooses to properly listen and give me the respect I expect. Once she is listening to her voice cues I stop and we go back to buisness as usual. She knows the difference in a kind "hello pay attention please" and "Your running because you can't act proper" she is a super intelligant horse. If I saw that she didnt know the difference I would find a better way to do things. Example is romeo doesnt give a flip why he is in the roundpen he just does it. Blah end of story. He doesnt enjoy backing up though to when he gets in trouble on the ground he has to back up. Simple as what works for each individual animal.
I also only believe that ground work fixes ground work, saddle work fixes saddle work. If the horse has handling issues on the ground, then the round pen is an option. If your only leading at a walk and the horse throws a fit and you make them run circles they go "Geez I was only having to walk...now im having to run. I shouldnt do that again if I don't want to run." If its an issue undersaddle it has to be fixed under saddle. Getting off and lunging for issues undersaddle just makes the problem worse. The horse sees it as "Hmm...I threw a fit and the rider is off now all I have to do is run a few circles instead of ride..."
So does lunging have benefits? Yes, its a great tool for respect, bonding, focus and on occasion punishment depending on the horse. Should it be used to fix all issues? Heck no...only ground issues...not saddle issues. Do your horses need to know how to lunge? Probably not so long as they don't give you an issue.