to invest a lot of time and money to pilot a project using one of the big-name commercial GUI tools. But if you recognize that you're going to have to do some programming even with a capture/replay tool, you may choose to skip the capture feature and code your tests directly using one of the free GUI test libraries.
You may decide that's all you need, and you'll have saved a lot of money and hassle. Or you may find yourself wishing you had features that are only available in a commercial tool, and then you'll approach the tool purchase decision much wiser. Or maybe you need to do some rudimentary link checking on your web site. There are several freeware tools that are up to the challenge. Most commercial web test tools offer many more features than link checking, and you can step up to them if your needs expand.
Maybe you're testing on a platform that is not well supported by the commercial tool vendors. I recently worked on a platform that was not supported by a single commercial tool vendor. If you have an open source test tool, you're free to apply the resources necessary to port a tool to your platform, without being at the mercy of the whims of your tool vendor.
One more argument in favor of freeware tools is the educational factor. You can make yourself more attractive on the job market by learning a new type of tool. It may be difficult to get licenses for a commercial tool just to play with it, but with freeware you can be up and running in minutes. Freeware is also well-suited for the classroom, and in fact, the open source culture has its roots in the academic research environment. I'm setting up a class to teach kids how to program, and I'm using an open source language so that my students won't have to spend any more money to be able to practice at home between classes.
There's room for both freeware and commercial tools in your arsenal. Because the freeware tools arena doesn't tend to have a big advertising budget, it's harder to get the word out about them. It's all about people telling people which ones worked best for them.