I didn't mean more fine-grained than polynomial. I meant between polynomial and exponential.
> On the other hand, for nearly all machine models, one can show that they can be emulated by another machine model with at most a polynomial-time overhead. This is seen as a strong sign that polynomial-time is probably what we are looking for.
> Another strong sign is that there exist problem classes (NPcomplete, coNPcomplete) to which every problem in NP/coNP can be reduced by a polynomial-time reduction (that such a class exists is IMHO a very surprising result). This is seen as another strong sign that polynomial-time could be a good idea.
These just say why we care about polynomial-time reducibility as a means for specifying a class. It doesn't say why we only look at polynomial and exponentials classes though, which is what i'm saying. There are infinitely many classes in between. Why just focus on these two?