Bloodstone Press
Explorer
Well, I hear where you're coming from, but this is true of lots of monsters. Dire monsters were created so that there were higher power animals for druids and rangers. The stirge is interesting because its only dangerous in large numbers (and is a low-power ability score damaging monster). The rust monster is the ultimate "threat to your stuff" encounter. Etc. etc. Most good monsters have an actual game design reason to exist. These monsters are popular.
You can add Hobgoblins to that list as well. Some time ago I ask Gary why DnD Hobgoblins are nothing like the hobgoblins from mythology. He basically said that he needed a militant humanoid that was tougher than an Orc but weaker than a bugbear. So he wrote up the stats for hobgoblins and slapped the name on them.
As for the phantom fungus, I haven't ever used one, but I have read the description and am familiar with them without looking them up. The main reason I haven't used them is that they don't seem to fit into the campaign very often. Another reason is that they are perhaps a little too alien for my low magic homebrew. But then, so are a lot of other creatures that I don't use very often....
On the other hand, I do occasionally run games in other settings that focus on weird, unusual monsters and abilities and such, and even then I don't use the phantom fungus.... Don't really know why. Maybe I'll add to an upcoming game and see how it goes...