That works until you get into anything complicated. Recently, I had a spreadsheet that our finance team built that provided different calculated values (for a multi table aggregation, nothing simple for sure) on Excel Windows and Excel OS X. This is not an uncommon problem - I've seen it a handful of times. Excel for OS X is at about 90+% parity with Excel for Windows.

This only bites you occasionally, but when you are a spreadsheet driven org (e.g. Finance groups in a company) you often need the performance/top-end feature set of Excel for Windows.