Excel (at least the 07 version I'm using now) has useful features that are not existant in opensource equivalents (pivot charts, etc), and some of the usability is much improved in Excel as opposed to say, OO.org Calc.
MS Access has no open source equivalent. Some small businesses would flounder without it.
If you don't need to share xls/doc files, then yes, more power to you, but if you do, you will run into problems using opensource alternatives.