The particular issue with wasps has do with if a 1HD creature can represent a hero (the PC's), then there isn't a lot of room beneath that of a PC to provide graduated distinctions between other creatures without resorting to fractions. For example, you could concievably have a situation where an adult (1 HD), a child (1/2 HD), a rat (1/2 HD??), a mouse (1/4HD), and a wasp (1/8 HD) all had the same hit points - '1'. Worse, they could all do the same amount of damage on an attack - '1'. So, now the adult farmer, the child, the rat, the mouse, and the wasp all are capable of killing each other with a single blow. Clearly the wasp ought to be represented by values much less than '1', but without doing some sort of fractional accounting - "Take 1/10th of a hit point in damage" - that's hard to represent. Also, even if you could find a way to represent it, describing what happens when the wasp is the target of an 'Enlarge Creature' spell isn't easy. That later sort of problem, trying to come up with rules that described changes in strength or HD, is what wrecked my attempts at a resolution to this in 1e (it involved treating numbers like 1d2-3 as representing a percentage chance to do 1 damage, rather than a 0 or a 1). Now that I've got more tools, I might come back to it.
For the moment though, the 'house cat problem' (what to do with numbers smaller than 1) isn't my big problem. Given creatures hit points as a result of size means that the wasp can't easily kill the mouse, the mouse can't easily kill the cat, and the cat can't easily kill the farmer with his hoe because they have steadily increasing hit points guaranteed. It's good enough for the moment.