Obviously Seventhsage's intent is humor, so I'm not going to be too critical, but for the sake of discussion: presumably the reason that the Andalites never are seen being helpful is due to a limited perspective on our parts, what to us it the All Important Human-Yeerk War might only be a relatively small battle between the Yeerks and a sufficient amount of Andalites to wipe out their current space forces around Earth.
They could be feeling that the Yeerks entrenched on Earth would be require more resources to take down than would be expedient for Andalite commanders looking to choose the best battles to engage in for minimal losses. It was revealed that "Z-space shifted" in some manner after the Andalites were defeat at the beginning of the series, so it would take much longer to get to Earth from current positions, meaning they would either send too many forces to wipe out the Earth-based Yeerks when they would be necessary elsewhere, or they would send too few and accomplish nothing.
They could be fighting in several locations throughout the galaxy to prevent the Yeerks from conquering various species that aren't quite as numerous as humans, but are much more formidable, if the Yeerks one on those fronts and got a hold of more species like the Garatrons and Leerans and an unimaginable variety of individuals that are more powerful than individual humans, then sent them to Earth to hold strategic positions along with Hork-Bajir shock troops, the diversity in their army may have made them much more unstoppable.
Still, I would think that what it would come down to is who conquered the space, so physical bodies would be unimportant to the allies of the Andalites and the hosts of the Yeerks, all that would matter is how fast they can build more ships and get out crews to man them, so you could certainly argue that it is a bad move strategically to allow Earth to fall and let Yeerks grow from thousands of viable host bodies to billions.
As far as the battle for Leera goes, the 'looking bad' of losing the land battle was just a feint (which probably took a lot of swallowing of Andalite pride to do), and even having traitors (as unimaginable as that was to them) on their side should have been no problem since they were supposed to lose the battle as a trap. The plan was a secret except those at the top (Assuming that they did have appropriate measures with the Leerans of the City of Worms to make sure that secret was kept), so the betrayal of regular commanders and captains shouldn't have been a problem. The actual problem in their plan was that the trap wasn't prepared and set-up in advance, and running into snags in rushed military operations is expected... But yes, it was certainly silly when their technology is super-advanced but they still set up things with such ridiculous drawbacks as was presented in that book.
