Within any system where an identity is cheap to create, that identity could belong to any user whom you already know. It is therefore in your interest to treat all users as you would treat strangers, until you have some form of assurance that they are not a user you know, but don't trust.
How can you know that a user is not an Alt of another user ? The only way to tell that two anonymous users are not the same is if they act differently enough to be considered different people. Every person in any "anonymous" system can be "fingerprinted" according to their vocabulary, interests, likes, dislikes, behavior, friends and knowledge. If two people have a great enough coincidence of interests, they are probably the same person.
Why is this important?
You should not trust anybody, unless you have very strong evidence that you can trust them with whatever information you give them. You are not making yourself much more vulnerable by replying to their messages, than by having a public profile, but you should be very careful what information you reveal to strangers. If you share 1 piece of information with 10 different strangers, how do you know they are not the same stranger? It can be extremely easy to piece together pieces of information over time.
Before you have established a relationship with someone, you have no evidence that they do not want to spam you with affiliate links, or slowly cipher incriminating evidence from you for some alleged crime, or even just a personal dispute. Because it's extremely cheap to create an Alt, you should assume the worst of every new user. This doesn't mean you should treat them badly, only that you should not trust them. As you get to know them over time, get to see their interactions, then you can gain some confidence in who they are and can begin to let down your guard. The personality that you see could always be a projection, but it's difficult to maintain a large number of psychologically believable projections over a long period of time, because those psychological projections will eventually "leak" into one another, and the separate "fingerprints" will be proven to be different fingerprints of the same hand.
 Other words for this could be: Alter, Pseudonym, Alias, Sock, Identity, Profile