Even though the new special stages look fun, kind of sad the old ones are being scrapped. Not to mention the old ones looked like a special stage location as opposed to a section of a level.
The old ones only look more like special stages to you because your mind associates special stages with the current special stage texture set. Imagine someone whose first experience with SRB2 is 2.1. They will simply learn to associate NiGHTS with special stages and it won't be awkward for them at all. The new ones were designed after the main zones because that makes it easier to make the stages look visually attractive. There's only so much you can do over the course of seven levels if you're always using the same texture set, not to mention that the new special stages are quite a bit larger in territory than the old ones.
I actually do think that some of the old special stages are quite cool. SS4, SS6 and SS7 are awesome and the part with the collapsing platforms in SS5 is also cool. As for the rest, it's just not very interesting to collect clusters of rings. What makes the ones that I previously mentioned so cool is that they actually require you to do platforming to get the emerald. In the first three ones and parts of SS5, what little platforming there is is vastly overshadowed by the act of collecting large clusters of rings over and over again, which is little more than an exercise for your direction controls. That doesn't have much longevity.
Out of curiosity, what made you guys want to scrap the old ones?
As far as I'm informed, the game was always supposed to have NiGHTS special stages and that's why NiGHTS was implemented in the first place. Warning: Unnecessary history lesson incoming, just because I can. I take no guarantees for this to be entirely correct.
The current special stages were originally implemented in Demo 4 as a placeholder because the system was much easier to code and the stages could be designed quickly. That was sometime in 2001. SRB2 2k3 (released in 2003, as you can guess) included a very basic and unfinished version of what is now Spring Hill Zone/ACZS, with about 20% of the track in place. The item placement was all messed up, with lots of items not matching up with the NiGHTS track and some of them even being stuck in or below the floor. The NiGHTS system was still in the process of being coded and there wasn't yet a reasonable way of placing items. (Objectplace mode was eventually invented so that Sonikku and SSNTails could place the items in runtime during a private netgames.) I assume that Spring Hill Zone was always intended as a proof-of-concept map.
By the time the Final Demo (1.01) came out later in the same year, the map was finished and the NiGHTS system was fully coded. What then happened is the reason why SRB2 took so long. Instead of designing stages for the final version 1.1 like they originally planned to do, they got caught up in bugfixing, hired a guy called a441 to rewrite the game menu and other things, added tons of new features and constantly changed the multiplayer maps around with each release. This lasted until 2006, when 1.09.4 was released. Then they finally remembered what they originally wanted to do and started mapping (I think Deep Sea Zone was the first stage they worked on). Then they figured out that they were making incredibly slow progress and hired a lot of community members to help them out. One of them was Rob, who started working on the special stages. This must have been around 2008. 2.0 included the first batch of content finished since 2006 and 2.1 will include the second batch, so to speak.
TL;DR: There were always supposed to be NiGHTS special stages, it just took until 2008 to get someone to make them. The current ones are placeholders.