Absolutely NOT.
If you do MX, I wouldn't even get a "Dunable" override tranny....
This tranny allows you to shift from 1-4 gear without using the clutch....but, you HAVE to pull the clutch in if you are not on the gas....and can not allow the tranny to slow the bike down or you will trash the shift forks.
I wouldn't even use a "Dunable" override for MX, you have too much to concentrate on for the track, rather than worry about what gear, clutch use, etc....
Even with a Dunable, which supposedly allows normal shifting up and down, still is a bad idea.