Here's why: 


  • 90% of spreadsheets contain errors;
  • Spreadsheet security is inherently weak;
  • No user read/write controls;
  • Compatibility problems between the people working with your file;
  • ...


Also: 

"Research has repeatedly shown that an alarming proportion of corporate spreadsheet models are not tested or controlled to the extent necessary to meet these obligations. Uncontrolled and untested spreadsheet models pose significant business risks, including:


  • Lost Revenue, Profits, Cash, Assets & Tax 
  • Mispricing and poor decision-making due to prevalent but undetected errors 
  • Fraud due to malicious tampering 
  • Systemic financial failure, due to overdependence"


SOURCE: http://www.eusprig.org/about.htm


Need more reasons? 

Have a look at this paper from Raymond R. Panko of University of Hawaii (5-10 min read) 

http://panko.shidler.hawaii.edu/SSR/Mypapers/whatknow.htm