DMR writes, "We live in a wonderful time when programmers can be so naive about the workings of Big Business."
Personally, I'm a programmer who's also been a business manager for eight years. I've used both Excel and professional accounting software (i.e., not QuickBooks, although QuickBooks is fine), and I can tell you: when I used Excel for operations (vs prototyping), it was because I had to. Excel can stand in for deficiencies in your accounting software, in a pinch. But it always requires vastly more effort to reliably perform the same reporting tasks in a spreadsheet.
As Jame Kwak points out in his post, spreadsheets are inherently fragile and unreliable. They need to be groomed and double-checked constantly in order to ensure you're getting correct results. Mistakes can be very easily introduced and easily remain undetected. The constant tradeoff is between spending a great deal of man-hours maintaining the spreadsheet, or in delivering reports with errors. (The JPMorgan debacle illustrates this perfectly.)
Contrast this with accounting software, which has reliable data models, automated ways of querying and summing data, and built-in checks for everything. If given the choice, I would always prefer to use accounting software for operations: the computer does far more of the work for you, and is constantly validating your input and keeping an audit trail.