I see it as a matter of degree. The fewer the "unique" combat encounters, and the more the turnover of encounters, the less it makes sense to put weight on the resolution of one of them in particular.Why on Earth wouldn't 4e lend itself to this?
I don't see this as the same thing as "theme"/"story" vs "mere colour" encounters. As you probably know, I'm not a big fan of colour/filler encounters. (Unless they're really pretty spectacular colour - at the risk of being self-congratulatory, I would put the one time I used a beholder into this category.)
For me it's more that 4e favours a lot of encounters, and even if they're building up to something big, or expressive of the theme/story of the game, no single one (except for the ultimate ones, maybe, with Orcus et al) quite has the "heft" to support the prep/prelim stuff.
But I agree if one was going to do it, the mechanical techniques (eg skill challenges etc) are there. But even then, what happens if the skill challenge or whatever fails? A game like BW supports the PCs coming into the big fight without the info/resources they need and getting creamed, because it has pretty robust failure rules and guidelines. I'm not sure that 4e supports this so well - I think it's closer to a situation where, if the PCs lose the end-game fight, it's game over - and that's probably another influence on my thinking about this.
I don't mind oddball monsters.its pretty easy to find accounts of campaigns that appear to have had this sort of issue where the GM wasn't terribly creative about building interesting scenarios and the game devolved down to a long list of 'fight to the death' scenarios 5x5 without much varying except the mix of monsters. Some of those will be interesting, but I think its quite possible for a game to become very cookie-cutter.
<snip>
I think though that some oddball monsters can help some DMs. I think that classic D&D had many such creatures, and 4e may have watered some of them down a bit too much, or just left them out entirely in a lot of cases. Certainly that can be colored as "focusing on the sweet spot of the design" but I never understood the mentality that having a Trapper, a Green Slime, etc would really be a BAD thing. Its not like you MUST use them.
I got good mileage out of a gelatinous cube encounter - they were one of the first Large monsters to appear in the campaign, and got to glide their corners over pits that the PCs were in danger of falling into. And I've used green slime, and a rot grub zombie, and a roper.
My favourite oddball/specialist monster has probably been the chained cambion, because its psychic chain ability did such a good job of making the players whose PCs got "chained" together get frustrated with and angry at one another (one was ranged, the other melee, and so they couldn't agree on how they should be manoeuvring while staying adjacent to avoid the damage) - which gave real concrete expression to the cambion's description as spreading its rage and anguish through its psychic powers - the players didn't have to roleplay that their PCs were feeling upset! I like a puzzle - as that was (how do they work together under the constraint of the psychic chains?) - that also engages the players viscerally and thematically.