Scrum I and II

It will be good to prepare for one exam rather than two exams.

Quite a statement...Thats is 1 "small" reason against a lot of other not to do this, AFAIK.

Personally, although I like the inspect and adapt approach on Scum certification, I think the current PSM I, II and III setup is very good and strong, and there is a huge amount of distinction between the levels, and a lot of reasons to keep it this way (at least for now).

I would disagree with combining these two exams.

It's been a while (almost two years) since I took the PSM I, but when I did, I felt that it was more of an "introduction to Scrum" exam rather than the first Scrum Master exam. And this may be in line with the fact that the Professional Scrum Foundations course leads into the PSM I exam. If anything were to change, I'd be in favor of renaming the PSM I to something more reflective of it as a general knowledge of Scrum as it's defined in the Scrum Guide and making the PSM II the new PSM I (and the PSM III the PSM II).

I'd also like the point out that the PSM II and PSPO I exams are much closer in difficultly to each other. Passing one of these two exams seems to reflect an understanding of the Scrum role that the exam is centered around. Plus, both (along with the PSD I) are labeled as Intermediate tests and required not only factual knowledge of the Scrum Guide, but some questions were about application of Scrum, which comes with some level of experience. PSM I is also called Intermediate, but the questions I remember were almost all entirely from the Scrum Guide.

I'm not a huge fan of requiring certifications for roles, but between two otherwise equal candidates on paper, the PSM II would far outweigh the PSM I in terms of giving me an idea of the person's skills and experience. In fact, I would encourage a broader number of people, including Product Owners and Development Team members to work toward the PSM I exam as a way of understanding the basic rules of Scrum and then toward the PSM II, PSPO I, or PSD I to gain a deeper understanding of the particular role on a Scrum team.

Add me to the "Don't combine" votes for all of the reasons that @Xander Ladage and @Thomas Owens listed. And a counter argument for your argument is that there is no reason you can't prepare for 2 exams in the same way that you would prepare for the single one you propose.  I gained enough knowledge to take the PSM I and PSPO I at the same time. In my opinion, the way that Scrum is intertwined in the roles makes it difficult to only focus on one role at a time.