Why in god's name would you include a clause that restricts your actions, unless your customer specifically requests it?
It's part of the package I deliver.
Even if the customer HASN'T thought of this possibility, the fact that I am willing to stipulate they won't be seeing me as a compeitor does a few things ----
It gives me additional credibility / honesty in their eyes
It gives the buyer more confidence in their decision.
It helps the project I sell to RETAIN/GAIN value, so when I'm selling future projects, I can cite examples of projects I'm started that have grown.
Plus, I'm a little tired of Craig's face after spending so many hours on that site.