Why in the hell would you ever consider shooting at 25k ISO? Just because a camera can do it, does not mean you should...no matter what, your images will suffer at that high of ISO...
The answer is quite simple - because without it, you'd get no photos.
Example? I got invited to my nephew's School Of Rock show at a local pub. They have live music there all the time, and having been a musician I know that there will at least be
some lighting if there's a stage. Boy was I wrong. So, here I was with my D800 and 70-200mm f/4 and to get any shot, even wide open, I'm at ISO 6400-12500 at a minimum, and even here I'm coming in dark. So, 1/2 the shots were at 25K.
Was there noise? Uh-huh. Could I clean it up. Pretty much - you could it was a high ISO shot, but probably not
that high. The important thing? I have shots of his concert.
My point is, no one ever goes in
planning to shoot at that ISO. But there will be occasions where it's your only real option besides, "Sorry, I couldn't get any pictures".