Why do you guys think that the Animorphs never used the Time Matrix during the war, or any other time? They knew where to find it, right? Maybe they could have used it to un-meet David, or stop Tom from getting the morphing cube... maybe let Rachel have another chance to take over the Blade Ship (take an auxiliary Animorph with her?).
And as another point, why did they let the Helmacrons run off with the Shrinking Ray? I mean, COME ON, it's a shrinking ray!?! You could just shrink an entire army of Controllers - Hork Bajir, Taxxon, Andalite, human, whatever - and put them in a tackle box or something. We know that Ax could operate it... Wouldn't that make it so much more convenient to starve out a yeerk? What bad thing(s) would have happened if they'd just stepped on the Helmacrons?
A Time Machine and a Shrinking Ray... matched with there morphing powers... things could've gone differently, don't ya think?