I view Microsoft Office as primarily a business tool, as I am sure the majority of users do. If I dictate a letter for a client including a sentence something like "you can be assured that your needs will go through a meticulous analysis" and my secretary types "thru" instead of "through", I want a squiggly red line to appear under the misspelling--but in Microsoft Word's dictionary, "thru" is acceptable. In my opinion, spell-checkers are already far too inclusive.