As a former salesperson and sales manager, I know how much effort it takes to close a complex sale. Although it took extra work and time to create a business case to prove the ROI of investing in my solution (an ERP system), I always felt the process was well worth it.
There are a few reasons I advocate using a business case as part of the sales process, particularly for long and/or complex sales cycles. One reason is obvious: a business case includes ROI calculations that prove to your customer the financial logic of investing in your solution.