@Gary E - I disagree with you. I think you're point of view is certainly valid but it also blankets all trial software as something that is made by mom and pop developers (at least that is what I got out of your post).
I for one use trial software that I have no intention of ever buying even though it is very useful. What is it, you ask? Visual C# Express 2010. Do I not buy the software because I'm a bad person? No, it's because I can't afford to spend $800+ for the commercial version.
I agree with Jeremy's point of view, use the trial as much as you can and then pay if need be. I understand that many people here create software for a living, however, if you offer this product indefinitely for free then you should really take a look at your revenue model. Nowhere did I see Jeremy state that you should use nefarious means to extend trials and I definitely do NOT support doing that, but if you are a developer and you offer a product for trial and do not put a limit on usage (whether time or features) then why should I "buy the cow when I get the milk for free"?? There is an easy fix to this problem, either time limit your trial or feature limit the trial.
Now I have a question for all of those in favor of paying regardless if it is needed. How many of you use Open Source software? How many of you that do, actually donate financially to the projects?
Wait until the software (or the company) forces you to pay. If the company didn't factor in someone using their trial version for ever and didn't expect that, they the messed up. The only other reason I would pay is if you have the money to do so and want to support the company so the software doesn't die or you need custom features.
If you are making money using a trial software program you should have already paid for it. You are directly benefiting from someone else's product. It is the correct, ethical thing to do. And just becasue the software has not forced you to pay does not mean you are not in violation of the license terms by using it for business purposes without paying.
Many of us here are software developers. We would like to continue supporting our companies and families by getting paid for our work. Some of us make mistakes and make our trial versions a bit too easy to continue using.
What actually separates using a trial copy of software for our own profit from looking on line for a serial number or key to convert a trail program into a full version? If your neighbor's child offers to cut your lawn and agrees to let you pay him later- is it ok to hide from him so you don't have to pay?
As long as trial software meets your needs, I see no reason to pay more due to any sort of altruism.
That said, most companies develop an idea of their current user base, their desired base, and an idea of what critical mass they want to support at an enterprise or white label level. Look at anti-virus suites: they tailor their products around getting momentum from a home user and give it away for free, but charge once you want to perform in power user or scalable environments.
The same goes for the flavors of Windows (Home v. Pro), or Commander Keen video games. If Chapter 1 is all you really want or need to play, why pay more?