I think there are a couple ways to use assessments for business purposes.
For potential clients I like to offer a free consultation that includes an assessment, this helps create a conversation around building a roadmap for success. I always re-assess my training clients as it only makes sense to have checkpoints with indicators.
Sometimes there clients that may just need you to help them stay accountable by testing their fitness. For these clients I charge a minimal fee, however these are the people that might just buy programming services from you.
With all that said, the hardest part is keeping time under control. Calculating and tracking assessment progress is very time consuming. Tools like http://trainermetrics.com make it easy quickly calculate assessments and share progress reports with my clients. You might give it a shot.