No matter if the client directly sells the software you built for them or just uses the software internally for managing / organizing his business, it's essential for clients to be able to make changes to the software without necessarily having to draw upon the person who originally built it.
I can't think of any reason for keeping the source code from the client other than shady and unethical business practices.
From my personal experience, there were only two occasions where the source code wasn't required and even in those cases I gladly supplied it.
I've been burned before by the original developers of an internal CRM software not supplying the source code (which in that case happened more out of laziness rather than malice). That original dev shop went bankrupt along the way and several years later the client called me because he needed a few vital changes to the software to keep his business running.
So, I took up the task of making changes to a Windows 9-something application that was written in VB5 and an Access backend without access to the actual source code. I did it using a hex editor and several work-arounds. It certainly was an intriguing challenge but nothing you'd ever want to happen to any of your clients' businesses.
Whoops.
Surprisingly, this works well enough that many clients opt for the non-exclusive license, and we get to re-use the code.