I think having the ability to send someone a PM would be really useful because:

• You might find someone discussing a topic on the forum, and want to ask them more questions without derailing the thread

• You could contact them with further questions on a previous thread they have made

• Casually chat with someone

A further system could be added to this where an icon could show where your friends have posted.
As well as people on your 'ignore' list, not being able to contact you through the PM system.