Unless their game actually involves more ground work, I don’t see the benefit. And though I’ve never done it, I imagine any really good bjj player probably has a high level of flexibility without actually training in doing the russian splits.
The crazy flexibility in this sport is on display in a lot of wmma. They get into crazy positions. But men and women aren’t built the same. And flexibility naturally deteriorates as you age, unless you specifically train it. Throughout all of my sporting experience the “hurdler’s stretch” was the closest I ever saw anybody do. And that was just a warm up stretching exercise we did to minimize injuries in games or practice. Most athletes do that.