Windows 7: Why Malwarebytes Is Not An Antivirus Pro.

Source -
Mac security – Why Malwarebytes Is Not An Antivirus Program

Who would honestly think it would be an AV?

It is an anti-malware/spyware/adware program,
While most attack currently are those styles of programs virus's are still quite prevalent.
An anti-virus program is written to detect virus's and in some cases common spyware,
An anti-malware program is designed to detect malware,
you get the idea?

Trusting one program to do everything is like trusting your routers firewall...
It just doesnt block/catch everything.

Multiple layers of protection are the key.

Internet,
Router fire-wall,
Your pc firewall.
anti-virus,
Anti-maleware,
anti-spyware,
end user.

From the end user to the internet,
Hopefully one of the layers will catch the baddies before they get to you and your information.