A cpanel account with JAILED SHELL is not the same as ROOT access. For the most part a Jailed Shell would only allow a person to do minor things on the server.
The whole point behind VPS is that it's your own environment allowing you to change how PHP operates, loading extra Apache modules, installing Java, sim linking directories, setting up custom scripts, interface with other locations, block IP numbers via a firewall, unblock those that need to be unblocked, etc etc.
If you provide cPanel and a VPS, and you are truely providing them a VPS (and not loading other accounts on there), then there really shouldn't be any reason NOT to provide the customer with root access, afterall, it's THEIR machine.