So on reflection, I'm beginning to think that a lot of the issues the Yeerks have happen when a Yeerk associates too fully with their host.
Taylor, obviously, is a prime example, but Visser One is an example as well. Visser One associated too fully with her host body, and the human concepts of love and parenthood (Yeerks, remember, should have no concept of parenthood outside their host, as they are destroyed by their mating process) changed her in at least one important aspect -- she bonded permanently with her human children.
But, hang on -- Yeerks should have no idea of mates, unless they have taken on traits of a host. Yet there was that one Controller who helped the Animorphs as vengeance against Visser Three because Visser Three killed his 'mate'. Is this just odd because we don't know a lot about yeerk pool society? Do Yeerks form an equivalent to shorm, something unrelated to actual mating? Or did that Controller bond too fully with his host and become involved with the host's concept of relationships and love?
And then there's the fact that I've been referring to these Yeerks as "he" and "she" -- where Yeerks, in their natural form, have either no gender or three genders. (As it takes three Yeerks to breed.) Indeed Yeerks will refer to eachother based on the gender of respective host bodies, without exception that I can think of off the top of my head, and in private, all-Yeerk areas too.
Many Yeerks seem to even have a gender preference; Visser One prefers female hosts, taking at least three female human hosts over her life on Earth. I believe she is referred to as female before taking a human host, so we might assume she had a female Hork-Bajir host as well. Do Yeerks consider gender to be another type of sensory information like colour? Is this why they develop a preference? Or is this all because of bonding too closely with the host again?
My personal theory is that Yeerks have no concept of gender before they enter a host, but that the gender of the very FIRST host they enter imprints upon their self-concept -- that is, that Yeerks are more affected than we suspect by the very act of controlling, and that it changes their... neural pathways, or brains, or whatever. The Yeerk would be imprinted as a gendered being, and would come back to the pool different -- not actually possessing a gender, but possessing a gender identity.
This could actually be used as an explanation for Yeerk intelligence and sentience. In reality, it would be highly unlikely that they could evolve such things in stagnant pools with no natural predators, no prey, and no outside pressures (for instance having to communicate with eachother and navigate complex problems to get to Kandrona rays). We will assume that Yeerks have the ability to interface with brains as a result of their ability to interface with *eachother* to communicate in-pool, however primitively. Here's another "This Is My Theory" Bit.
Say, theoretically, that there was at one time on the Yeerk planet at which a primitive but intelligent being roamed the surface above the pools (I don't know if Gedds count, let me know if they do); the volatile nature of Yeerk Planet weather alone does give them some evolutionary incentive to need intelligence. Possessing curiosity, it scooped up a Yeerk from the pool and, seeing movement, put the Yeerk to its ear to see if it made noises also. (Sounds silly, but I could see a toddler or something doing that.) The Yeerk, sensing neural activity, travelled up the ear canal. Eventually, the Yeerk discovered it could control this creature, but it also discovered real, however primitive, intelligence. This being the first host, the Yeerk was imprinted somewhat upon it; not only in concept of gender if applicable, but its neural pathways restructured severely. Later it left the host to return to its pool, and eventually bred. Either its DNA was somehow restructured (unlikely) or it still had the DNA of that first host on it -- either way, the resulting Yeerks were left *aware* that they could control other creatures, and more intelligent for the essentially mutant DNA. This process could happen with a great number of Yeerks. The resulting Yeerks would be at an evolutionary advantage, as they could leave their pool and travel long distances to another if it threatened to dry up.
I dunno, it's a bit silly, but it's an idea I've had. (Another, probably more reasonable idea suggests that Yeerks at one time had far greater evolutionary pressures, and that 'sitting around in stagnant pools their whole lives' is a relatively recent development -- just as 'fast food joints and no real predators' is a recent development in human evolution.)