You can make good arguments against using free security tools. They can contain their own adware or malware (a means for the programmer to recoup their investment) and they can be inefficient, untrusted and open security holes of their own.

For the first argument, I suggest weighing the popularity and usefulness of the tool as well as mitigating its use by scanning the tool with trusted software.

For example, I use one freeware tool as a form of layered protection, knowing that it comes with adware. It's a popular and effective tool, so there are benefits. Further, I mitigate using the tool by scanning with a premium anti-spyware tool, which catches the tool's adware and disarms it.

In this way, I get the benefits of the tool without the harm.

As for the second argument, there are popular freeware tools that are simple enough to examine yourself, safe and effective, and come from known-good sources.

For example, I regularly update the hosts file from http://www.mvps.org/winhelp2002/hosts.htm in order to block known-bad sites. The file is a popular tool that comes from a Microsoft Most Valuable Professional (MVP). The source is good and the tool is good.

Logic dictates that a simple hosts file that I can open in plain text and read, finding only known-bad site links mapped to the local home address 127.0.0.1, is not a threat.