<p>Developing websites are time-consuming. To improve productivity, I would code a prototype to show to our clients. I don't worry about making the prototype comform to the standard. Most of the time, our clients would approve the prototype and give an unreasonable deadline. I usually end up using the prototype in production (hey, the prototype works. No need to make my job harder.)</p>

<p>I could refactor the code to output valid HTML. But is it worth the effort to output valid HTML?</p>

Yes. It's hard enough trying to deal with how different browsers will render valid HTML, never mind trying to predict what they'll do with invalid code. Same goes for search engines - enough problems in the HTML may lead to the site not being indexed properly or at all.
I guess the real answer is "it depends on what is invalid about the HTML". If the invalid parts relate to accessibility issues, you might even find your customer has legal problems if they use the site on a commercial basis.