Scenario: 

Create a table (with or without header) with two columns and three rows.

I will attach a test document if possible, but it is easy in theory.
If you would number the six cells (2 columns and 3 rows) in the way an european
write, you get something like this.
1,2;
3,4;
5,6;

You may make a cell merge in Star/OpenOffice for cells 1 and 3. But if you try
to do it for 4 and 6, you receive an error message that, the cells are too
complex to merge.

The reason is, instead of using a rowspan attribute, the writer creates a
subtable for the cells 2 and 4, which lay aside of the merge.

I had such a scenario with the two mentioned cell merges, and it is quite
possibe that any customer sooner or later will have the same.
The only way for me was to edit the XML code manually.

I would suggest to raise the target of this 'enhancement'.