Aside from the fact that the only stage on which the Mariinsky Ballet will perform in St. Petersburg during the reno -- unless Gergiev plans on taking over another stage that is raked -- it makes more sense for the studios in which they practice to be flat, at least in the short-term, so the dancers don't have to go back-and-forth. Long-term, though, I don't know how hard it would be to re-adjust to the rake after years of flat floors. (One of the reasons the Bolshoi reno took so long was that they found that the foundation was rotting, and who knows what they'll find when they open up the Mariinsky and how long the renos will take.)
If the raked stage is closed down, and the company touring, wouldn't having a non raked studio better for the duration of the reconstruction?
The younger students need to get their foundations for performing on a raked stage; the ones closer to graduation will perform exclusively on flat floors for the beginning of their careers if they join the Company.