As a developer of custom business webapps, I have been in this situation many times and I desperately wish I could take this advice. Really, I do.

But when a client makes a request like this, often unaware of what may actually be easy or fast or simple or consistent, I can't say no to it. Saying that we need to take more time to write tests and document it properly would likely come across as similarly unprofessional and reflect badly on the company. And the consequences are every bit as bad as you say they are: the code rots and becomes volatile and obnoxious to maintain.

That's why it hurts to read this post--it feels so painfully true. I'm lucky in that at my company, my project managers are coders themselves who understand the programmatic constraints and the danger of software rot--but my clients usually aren't. I have no solution for this; it's a fine line to walk.