Honestly, the lenght isn\'t that big of a factor, because you\'ll go through it again and again...I mean, take DMC for example. It was, in basic sense, a very short game. However, if you wanted to become a master at it and unlock everything, the game\'s lenght became much longer...Zone of the Enders 2 is the same way...
Once you beat the game, you unlock four different versions of your Mech - each different. Then you can raise the difficulity, if you want. Find the ex-missions, get S rankings on the missions and then there is always the VS mode.