This is pure gold on why I don't subscribe to the whole "if it's not meant to be fought... it shouldn't be in a MM." philosophy. It pre-supposes what actions the PC's will take with what creatures, and as each campaign is individualized... how can that be determined by anyone but the players themselves. Personally, until 4e, I didn't consider unicorns something heroic PC's should fight... but hey different strokes for different folks.
As a DM my purpose is not to determine what will or will not be fought by the PC's, but instead to simply provide situations where they can react however they want... if that means fighting something that's not a threat more power to them... just like if that means negotiating with a "monster they are suppose to fight"... I'm not going to make them fight it.
I don't think it should be in the MM if it's "meant" to be fought, but if interaction with the PCs in combat would be meaningless then it shouldn't be in there.
If it can present a reasonable combat challenge to a PC at some point in the game then it should have combat stats. If not, it doesn't need them.
If for instance the thing has no hope of doing anything against the PCs, can be eliminated effectively by the PCs stepping on it or something... Then combat stats just get in the way, and confuse things.
Not having combat stats doesn't mean the PCs can't fight it... It just means they win, with pretty much 0 effective resistance.
That said, I wouldn't be opposed to a book of "encounters and challenges" that contained things like traps, terrain, diseases, poisons, skill challenges, and etc- ways of interacting with some stuff as a "challenge" that isn't combat all in one place.
Campestri are in the game already, but since they don't present any sort of real combat challenge they don't have (or need) combat stats, or a spot in the MM. Instead, they interact with PCs in another way, effectively as magic usable terrain, and the stats are supplied for that interaction.
Mosquitoes would be something I'd do in a similar fashion... There's no need for combat stats for a normal mosquito, but as a chance to catch Malaria?