That is true - and that is the crux of the problem with them. Not their concept, but the lack of guidelines to use them smoothly with the fiction and how to judge situations where abilities and ideas bypass checks, Also, mismatching DCs (e.g. for climbing up) are a DM problem. If climbing is easier than the skill challenge, then the skill challenge should be easier or the wall they're climbing up should be more challenging.
No, the point of a skill challenge is to give XP out for a non-combat challenge. In order to be able to predict the correct amount of XP to give out, you need DCs that don't change. That's why the XP for a skill challenge is based off of two things: The level of the skill challenge and the complexity. The level of the skill challenge sets the DC for all the skill checks and the complexity determines how many times you need to succeed without failing.
These two factors allow you to say "A level 10 party attempting a complexity 5, level 15 skill challenge has almost no chance of succeeding, so they get a lot of xp should they succeed."
However, if you lower the DC of the skill checks, even if its only one of them then you are making the skill challenge easier. If the DC is 18, then all the skill checks in the challenge have to be DC 18.
This particular problem comes when there are two conflicting ways of determining the difficulty of something, in addition to narration issues. If the PHB says "Hey, a rope with a wall to brace against is DC 15" and the DMG says "All DCs in a skill challenge are the same, based on the level of the skill challenge" then as a DM it's difficult to narrate this wall as DC 18. You can say "Well, this is a little more treacherous than the standard wall, lots of sharp rocks sticking out, making it 18 instead of 15." But players now have the idea in their head that all walls with ropes are DC 15 because it's written in there. It creates a bit of a disconnect. Plus, many DMs don't like to go into a large amount of description. So they are likely to summarize the description as simply "a wall with a rope."
In addition to that, there is a severe problem with narrating things when the DCs have to all be 18 but the players decide what they are doing. If they come up with a decent idea but the DC should be extremely easy, does the DC become easier for that one roll? Does it count as a success at all? Or does the DM have to come up with some convoluted description to match the DC? And how many times can one DM come up with descriptions for DC 30 climb checks before it becomes silly?
That is also a problem with the page 42 DC master table and how people interpret it (i.e. the wrong way round, DCs don't scale to the level, the narrative should scale to match the level-appropriate DC).
The narrative should match the DC, I agree. Which direction one comes at it is rather inconsequential. If a DM says "I want this to be a challenge for the PCs, so I'll look up what a challenging DC should be for their level...Hmm, 28. Alright, this climb check will be DC 28 then. I'll just describe it as extremely dangerous or something." it's just as valid as if the DM says "Ok, this cavern has sharp rocks sticking out of it, it has outcroppings, its super slick with moss and water. I'd say it's DC 28."
I'd hazard a guess that the table is used more often for the former than the latter. I know that's how I used it. My last DM based all the DCs in his game off appropriate DCs for our level. We were asked to use it that was for DCs in Living Forgotten Realms adventures as well.
The idea being that if a DC wasn't level appropriate than it wasn't worth including in an adventure. DCs much lower than the DC for the PCs level were pretty much assumed to auto succeed and DCs much higher than the appropriate DC shouldn't be included in adventures because the PCs are the heroes and should always have a chance of succeeding.