There are many good reasons for choosing open source over proprietary software, notably the freedom it gives you, and the security that comes from allowing anyone to look for bugs.
Here's an amazing example of what by contrast can happen with closed source in this regard:
Microsoft took the unusual step today and skipped patching one of the vulnerabilities addressed in its monthly security update, saying that crafting a fix was "infeasible."
The omission leaves users running Windows 2000 Server Service Pack 4 (SP4) vulnerable to attack.
Earlier today, Microsoft delivered five critical updates that patched eight vulnerabilities in Windows, including one that the company won't bother fixing in Windows 2000 Server SP4. The operating system's support doesn't end until July 2010; until then, Microsoft was supposed to provide updates.
This is extraordinary: it means that Windows 2000 becomes abandonware – no longer fully supported by its manufacturer, and left with a critical security flaw that makes it pretty much unusable in many circumstances.
Contrast this with open source. Even if a company providing support for GNU/Linux decided not to provide a patch for an old version, end-users always have the option of writing one themselves, or paying a third-party to do it.
Now, that may not be totally satisfactory, but at least you have that possibility. Not with Windows 2000: Microsoft has decided not to patch that flaw, and there's absolutely nothing you can do about it...apart from junking it, and moving to an operating system that puts the user in control.