After the Andalites won the Battle of Leera (book 18), I expect that they employed the mind reading Leerans to root out all traitors in their midst. After the war, they would use Leerans to read the minds of suspects to determine their guilt or innocence.
While I am almost inclined to agree with Dpsb and RYTX . . . this idea DOES actually explain why the "oh noes the Yeerks are beginning to infiltrate the Andalite homeworld" plotline was so very suddenly dropped. Which was something I've always wondered about.
Maybe the Yeerks actually
were moving against the homeworld, at least in terms of collaborating with traitors like Samilin. But before they could plan out an actual full-scale invasion, the Andalites formed a small truce with the Leerans. They wouldn't allow them to 'scan' just any Andalite whenever they wanted, and of course the Andalites would only pick a very small number of Leerans that they ABSOLUTELY trusted, but then they might let those Leerans 'scan' Andalites in very high-ranking positions in the military (those particular Andalites that, if they were traitors, would ruin everything, like Samilin did).
Again, Dpsb and RYTX have valid points. The Andalites don't like to trust other species. But desperate times call for desperate measures.
. . . Or maybe they could just screw the whole 'trust other species with important tasks' thing and just morph Leerans themselves.
Yeah, that would probably make a lot more sense from an Andalite point of view.