Is there any reason not to tile right over a freshly-laid block-and-beam subfloor?
This is a first time I see such a way to build a subfloor, it's not common in a country of my origin. It seems, however, that concrete blocks are not simply a filler here (as in other types of ceiling/floor structures), they can bear loads and play load-bearing part.  

That said, I can still think of several reasons to put the concrete layer on top of the structure:
1. the layer will fill all the cracks and defects of the structure (You asked about tile mortar - it's more expensive to use it instead of a concrete)
2. the layer (properely laid) will level all the subfloor to stand for a good base for tiles
3. the layer will distribute dynamic loads better than a floor without it to prevent single-tile cracking
4. if it's a subfloor over the ground, then You will need some moisture insulation and thermal insulation (ask google for proper layer order), I presume that it will go between a structure and the concrete layer  

I encourage other SE users to correct me if I'm wrong here.