Creating good spreadsheet project requirements takes time, work, and the participation of both the client and the developer. It can be very tedious work and the benefits - and consequences - failing to do a good job are not immediately obvious.
The benefits of having good requirements are:
Fewer spreadsheet defects
Less time spent on development rework
Fewer unnecessary features in the spreadsheet
Faster spreadsheet development
Reduced scope creep
The consequences of not having good requirements are:
Rework - redoing something you thought was already finished and the extra cost you incur from having to do redo and refine requirements that were not properly written in the first place.
A rework cost is what you have to pay when you have to go back and fix a problem in your spreadsheet. Without careful planning to minimize the need for reworking the project, your development costs can be 30-50% higher than projected. Of the total rework costs you may incur, it is likely that requirements errors will account for 70-85% of total rework costs.
Unfortunately, it is all too common that requirements errors go unnoticed until the spreadsheet application has been implemented. At that point, your cost to fix the error is much greater than it would have been if you had discovered and fixed the error during the requirements stage.
So take the time to properly plan your spreadsheet application requirements. Make sure all stakeholders participate and have a voice in the project. It will be well worth whatever amount of time and money is required. It will cost much more later on if you do not get it right from the beginning.