I need to express this in FO:

| | | | | +----------------------------------------+ |
| | ABC-DEF-0521 45 | |
+----+----------------------------------------+----+

It's a page footer with two blocks in, one for page number, one for a
document-id. I use to solve these cases with tables. In this case I'd have
three cells where 1 and 3 have equal sizes, so the page number can be
centered. The question is if there's a more effective way to have to blocks
beside each other than using tables?

There's a lot of code for something
that appears to be simple.

A wooden rod is simple too, but it ultimately consists of
an awful lots of molecules arranged in a very complicated
manner. You can go to subatomic levels if you want.

The point is: XSLFO provides basic blocks for describing
a 2D layout. Most stuff embodying higher level concepts
are stripped away and mapped onto combinations of more
basic blocks. Effectivity in the sense of "number of
elements to describe a certain layout" was not a concern.

From this point of view, fo:table is a bit misdesigned,
because it has headers and footers to make the description
of some real "high level" tables easy but is also the
only way to describe a more general "grid" layout block,
which you just encountered.