Why should one choose commercial security software?  

Well you shouldn't...

If an experienced intruder wants to get into your PC they will find a way to get around the commercial security software so you might as well use the free software. I never had any problems using those programs and never considered paying for security software. Truth is you can never be 100 % secure if you are connected to the Internet.

I really don't think commercial security software offers much significant advantage over free ones.The support will be better,that is if the software doesn't work properly then the company will help you if you contact them which is not the case with free software.

In today's age,we are seeing almost of kinds of softwares available for free.From operating systems to games,many are freeware now.In many cases,the free one's are even better than the commercial ones.Security software is no different.

I think you should try out the free softwares and only if you are unsatisfied should you buy a commercial one.

i think as free software are available easily, experienced one gets time to find the loopholes of it and to break it. So commercial one are more secure in a simple way.