Oversplits can be useful but are of no greater value if the gymnast cannot do full splits in the first place.
It's not like you are putting more pressure down than what they should already be placing on their splits if they are doing them correctly and pushing for them.
Sometimes it may force them to go down lower than they would otherwise go. Then we have a problem of a lazy gymnast.
However, with some gymnasts, especially those who are very tight, I don't think they can exert enough force on their bodies to compensate for how tight they are.
As well, there is some evidence that excessive flexibility cuts down power potential.
Sometimes, it's not that they do not have passive flexibility but not enough active flexibility. They may be able to do splits on the ground, but not splits in the air so they need to work on their active/dynamic flexibility.