So I asked myself: Why were they stuck in a Monster Manual and given stats? What gameplay purpose might they have served? What were you supposed to do with these things at the table? Why did someone go through the effort of writing up a whole page of fluff about singing dancing mushrooms?
And here's the answer I came up with: Monster Manuals aren't just lists of things to fight. Or at least, they weren't, up until 4e's MM's. They are lists of interesting things to interact with.
I think you might be giving whoever put the campestri in a Monster Manual too much credit. They originally appeared in "Old Man Katan and the Incredible, Edible, Dancing Mushroom Band", from Dungeon magazine back in the day. They were comic relief in the adventure.
There's a very good chance that whoever put them in the book did so because they were told to draft all the new monsters from Dungeon for the book, not because of any greater philosophical statement. That's also why some of the monsters from Fiend Folio seem so bizarre. They originally appeared in adventures where they made sense, but are fairly pointless outside of that context.
Now, all that said, I think there's plenty of space in the game for monsters that are interesting outside of a fight. One of the things we tried to do in MM 3 was give more reasons for PCs to interact with some of the monsters, such as the chitines, outside of combat.