Once upon a time, all gamers were new gamers and many monsters such as the rust monster and ogre magi have their origins in those early days invented by some of those very new gamers who were running them for other new players. Sometimes these new ideas worked out well, sometimes they did not.
FireLance has no difficulty using oddball monsters as he can gauge at a glance the effect they are likely to have in a given situation. As he says, "I'm a sufficiently experienced DM..." how was that experience gained? How do the new gamers become experienced?
By trying things and finding out what works and what doesn't. Not every game will be a perfectly created storyline, tightly run, action packed and thrillingly played - sometimes a game flops. That's OK. It's no one's fault - mistakes are expected of everyone as part of human nature, we are not infallible. We learn from those mistakes to make the next game better and more enjoyable - in the process taking steps to become that "experienced DM" or "experienced player". Those are the experiences that make us experienced.
It seems to me that American culture, at least, currently has difficulties with perfectionism.
We protect our children to the point where they are unprepared for the world because they have never experienced it. We're so paranoid about germs that we weaken our immune systems by minimizing contact with germs to such an extent that they fail to form antibodies to common infectious agents, while at the same time promoting the evolution of those bacteria towards resisting those antibacterial agents we use to fight them. We don't want our children to make the same mistakes we did, in fact, ideally we want them to make no mistakes. The problem with this is that we learn a whole lot more from our mistakes and failures than we do our successes, so in preventing children from making their own mistakes we also prevent them from learning from them. The problem people have with oddball monsters and "balance" and "warning labels" and "handholding", I think, is an outgrowth of this difficulty with not wanting our "children" - i.e. the next generation of gamers - to have to make the same mistakes we did and have to go through the really bad gaming experiences we did when we were young, so they can just have the fun part. Of course then they can't learn from those mistakes to learn to judge the difficulty of a particular encounter on their own and instead rely on a Challenge Rating to do it for them, because they've never done it another way. While a Challenge Rating is a good tool, it's no replacement for that gut feeling that tells you when something is likely to work or not.
This also seems to interact with our current seeming desire for instant gratification and general lack of patience. We don't want to boil potatoes, and mash them by hand when we can take flakes, add water, nuke them for 3 minutes and stir.
None of this is intended to say any of these trends is good or bad, simply an observation of general tendencies that I see in American society in general and how they interact with the hobby as a whole and this topic specifically.
But that's just my thoughts, maybe others think differently.