Being able to merge rows only or columns only within a selection.
On selecting a set of cells (multiple rows and columns), having the ability to merge all of the cells in each row or each column would be useful.
I have to do this when setting up table headings. Where the headings may have a master and sub heading for some columns, the rest I have to select and merge cells in all of the other columns one column after another to get this affect, so that centering in height or width then looks better,
Mike McClory commented
Spreadsheet to show concept