Protection of formulae : How often have you forgotten to protect the cell contains the formula that took you hours to test and perfect. It takes one instance of honest human error, and that formula is gone.
Inconsistent data : This becomes most apparent when trying to find a particular record or cell, based on the contents. If a space is incorrectly added in the wrong place, the search or filter fails.
For pure data analysis, the spreadsheet is, indeed, king. To a point...Despite all of these extremely powerful features, the spreadsheet has limitations, which tend to manifest when you least expect it. Often, Murphy's Law applies and the spreadsheet falls over when you are right in the middle of a major undertaking, like month-end or meeting a tight deadline.
You can also create a calendar with a spreadsheet application and keep it within your budgeting workbook. You can list dates bills are due and you can also use the calendar in accordance with your budget to plan for special items or vacations. Having everything in one place will make it easier to manage your finances and plan for the future.