The fact that the 'malicious users' (as Symantec calls them) have the access to the program source code, dramatically increases the chances of building successful exploits. Knowing the loopholes in the security verifying procedures, a hacker can work out a way to bypass any anti-virus barriers. This, in its turn, can result in the hacker taking over the control of the user's computer.
That's why Symantec recommended the users of pcAnywhere either to stick to the best practice procedures (prescribed by Symantec) while using the program, or stop using it at all.
The funny thing is that the Lords of Dharmaraja hacker group, who are blamed for the incident, alleges that the only thing they did was hacking a server of an Indian secret service. There they found the source code of Symantec's program. Apart from that, they 'discovered within the Indian Spy Programme source codes of a dozen software companies which have signed agreements with Indian TANCS programme and CBI.'
If these words are true, than we should wait for a series of anti-virus programs hacked by the Indian cyber-criminals.