Viruses as products have several good sides:
- there's no advertising, pyramidal sales scheme or mail-in-rebate scheme about them; nobody will telemarket it or yell at you from your radio or TV to buy it
- they work
- they are distributed at no cost - nobody charges you shipping & handling
- you don't have to sign any sort of EULA to get your box infected
- you don't care whether they are proprietary or open-source
- there's no monopoly
- you don't care whether the manufacturer will be in business after a number of years, both your investment and cost of ownership are zero
- you don't care in which language it was written
- there's no licencing scheme so you don't have to pay a license managing app/person
- there's no disclaimer with a megabyte of hardly legible legalese.
- no learning curve
- no need to give your data over the phone six times until you get to the person who knows what you're talking about
- no fancy GUI to confuse the end-user

Downsides:
- A virus is often obsolete and too compatible with your antivirus software
- It doesn't always work; it may choose the days when it feels like working
- When it works, it's often not what you want (but hey, so is the case with most of the other software)
- No help file
- Documentation is very thin or absent
- no fancy GUI to amuse the end-user
- your OS may not be supported (tough luck, Linux crowd)
- your email client or browser may not be supported (too bad for their users)
- it forces you to buy or keep downloading upgrades for other components of your system
- despite the abundance on the market, you don't have a choice.