You know, they did a similar thing to another cartoon. It was a little show called Ren and Stimpy. It was remade into an adult cartoon with no censors. The result was ulitimate suckage with even the fans of Ren and Stimpy hating it, and as a result it was canned after 6 or 7 episodes.
Mlp is not the type of show for that. It's attatched to a brand that for years has been marketed towards 5 year old girls and has been recognized as such. Rebooting it could be damaging for their IP, as parents would probably lose their trust in the product to protect their kids. Sure, making Samurai Jack adult worked, but the difference is that show was standalone and not a franchise and came out when all the fans of the orginal show grew up (not to mention only airing late at night when kids are sleeping).
Besides, we already have an adult version or MLP. It's called Pony.Mov.