Advantage?
Let's assume a pitching distance of 43'. Regardless if she leaps or drags, if a pitcher replants her pivot foot 3' in front of the pitcher's plate and pushes off again before delivering the ball, she has effectively pitched from 40'. So, the "advantage" question is pretty simple: Which distance is more advantageous for the pitcher; 40' or 43'?
Getting it called?
Identifying a replant is actually fairly simple when viewed from the side, such as from first base or third base. However, from a head-on point of view, it's nearly impossible. Remember that the illegal activity is the actual replant, NOT the symptoms. However, the symptoms in this case are very telling if you have studied fastpitch pitching mechanics. Nearly every non-replanting pitcher has a smooth continuous flow in her forward motion. However, a replanting pitcher will nearly always have a "hitch", or momentary stop in her forward motion. This "hitch" is generally where the pivot foot has completely stopped it's forward motion due to being "replanted". Another symptom is the hole created by this repetitive replanting.
So why isn't replanting called on a regular, consistent basis? I simply don't believe that these field umpires cannot detect the infraction. IMO, replanting is so epidemic in Ohio HS and travel ball that calling it on a consistent basis would be like calling "true" strikes in a 10u rec. game. It's an unspoken game-flow thing. Umpires must take liberties to keep the flow of the game moving, or else games would be seven hour ordeals!
Solution?
So, why do all these kids replant? I don't believe for a minute that they are trying to cheat. They just honestly believe they are not doing anything mechanically wrong, and they have been doing it so long that it feels natural. Numerous illegal pitches in games are a symptom of an underlying mechanics problem that probably started early in the kid's pitching education. A common problem in young kids learning to pitch is the tendency to lean forward and guide the ball. In trying to teach them to stay balanced - neither forward nor back - an instructor might stress to "keep your weight back". Combined with other mechanics and timing flaws, this can result in putting too much weight on the back side pivot foot, causing it to stop and replant. An instructor who continues to ignore obvious flaws, and the kid who refuses to work hard to correct them are the ones at fault - NOT the umpire who is simply doing what they are trained to do. The solution is for parents of pitchers to extensively educate themselves about what legal pitching mechanics look like, and insist that any instructor teaching their daughter adhere to these principles.
It is insanity to blame umpires for calling replants! When illegal pitches are called, it's NOT the umpire's fault. This "fault" started years before with an unqualified pitching instructor and ignorance of the rules. My suggestion is to parents: Don't embarrass your daughter by getting upset at an umpire for recognizing your daughter's illegal pitches. Take a proactive approach and MAKE SURE she is pitching legally before hitting the field. She will thank you for it!