After all, the differences between hand-crafted difficulty levels still rely on the whims and gameplay balancing skills (or lack thereof) of the map's author, while the ITYTD changes are always there for you.
Well, there really isn't such a thing as gameplay balancing skills, simply because there are no set difficulty levels to target. The closest you can get is saying something arbitrary like "HMP should always be like Doom 2's HMP", but even that is ineffective and, more to the point, completely bland and it leaves out the fact that player skill doesn't improve in increments and doesn't have any specific boundaries. This thread reminds me of Battle for Wesnoth, which has a bunch of campaigns each with a difficulty level label such as "For beginners", "For advanced players" etc. Then in addition to that, each campaign has a number of difficulty levels to choose from. So is the hardest level of a beginner campaign easier than lowest on an advanced campaign? Is it equal to an advanced normal level? Do these difficulty levels take into account the player's varying skill with different races? Or skill fighting against certain races?
No, that information is, all things considered, mostly useless, as are difficulty level labels useless in Doom.
From the sound of it a lot of people expect certain skill levels to play in a specific way, like "HMP gives the most balanced experience", etc. Well, how do you know? You're assuming that the wad's difficulty levels relate to some arbitrary expectations you have from earlier wads, most probably the IWADs, but there's no guarantee that's going to happen. And you know what, that's a good thing. Because if we were stuck to some ridiculous expectations of what skill should be, we wouldn't have the same level of speedrunning we have today. Nor would we have the more extreme wads aimed at people who are better at the game than others are. The whole Doom "scene" is enriched by that, and anyone thinking that a specific difficulty will always be the best for them is just being silly.