Just mechanics-wise, the Spin-Dash is outright useless in a three dimensional environment. It was one of the most noticeable things about the Adventure games: suddenly, in a wide open environment and a less linear environment, the fact that you had the stop and wait for a moment before getting speed you need to do things became more sluggish than necessary. It works a lot
better in 2D - most things about Classic Sonic do, that's why he excels in that environment (it's another reason why he's a lot more platforming heavy as well, singular focus on speed isn't quite as easy to pull off entertainingly in 2D as it is in 3D).
So to continue reimagined Sonic's movement: they made running more important, since accuracy over a single burst of linear speed was far more useful. After that, they just took it the extreme: they returned to linear design, but rather than bring back a mechanic that works better in a two dimensional system and feels sluggish in 3D they merely implemented a system that grants speed immediately in order to focus on aesthetic and speedy gameplay. I'll be the first to complain about how the boost was implemented in Unleashed, but they proved that they could improve it with better level design in Generations.
Personally, I prefer the way Colors did it - basic running and platforming, with boost and powerups only being useable for short times and ultimately used for very specific purposes. In my opinion, the next Sonic game should use both wider and linear environments, and feel fast but require neither boost nor spindash in order to do so. But I'm not about to belittle gameplay features and mechanics I don't like as "gimmicks"
because I enjoy something best.
These things happen in game design - not everyone's skillset is as universally adaptable as Mario's. If it makes you feel any better (which it probably won't), sometimes while boosting in Generations Sonic does do the spindash animation.
"My final prayer: O my body, always make me a man who questions!" — Frantz Fanon