As a programmer myself, I have worked several projects that made me out to be an idiot (I can do this on my own, I don't need any help, thank you) due to this kind of "protection." I was only privy to specific areas and had no knowledge of the effect it would have on other areas.
For example, a simple change to an .htaccess file breaks something that I was not informed of, didn't even know existed, and the reaction was that I should have been psychic enough to "just know" it would break something.
If you hope to get the best work out of your programmer, complete and total disclosure is absolutely essential, especially if the site is complex and large. They need to be made aware of every nuance and detail so a seemingly harmless action doesn't initiate self destruction.
And yes, there are a lot of B.S. programmers out there, ones who, rather than take the time to review the code, understand it's logic, work with it, will just say it's a pile of outdated amateur crap and needs to be built from scratch (this should be your first red flag, whether or not the statement is true.) So your concern is legitimate.
It boils down to one thing: reputation and trust. Find someone who has a history. Review it. Ask if you can speak with some of their clients, personally I'd completely agree with this if asked. Like it or not, you need to trust your programmer and system administrator as you would trust your spouse, if not more.
If you do find someone of this caliber, after a common NDA is agreed and signed, your first step is to have them LOOK. Don't touch. Look, build a report. Pose some questions/problems/tasks you absolutely know the answers to, gauge their response. See if they overlooked anything you think they should have caught. See if they have ideas you'd never thought of, which would make them a very good asset.
Then if it all works out, marry them - err, write up a contract. Same thing. :-)
A reputable programmer would never risk "stealing" your web site in exchange for their reputation. Personally, the only things I gain, other than compensation for services, are the new ideas and knowledge I learn by working with a new project.