MBTI was designed as a tool to type yourself, not others, and providing a way to interact with people who seemingly already know their own types. Misidentifying yourself is always a danger, but mistyping others is more likely - you try to type based on impressions, but you can't really know what cognitive processes others use. You may mistype a stressful person frequently using his/her shadow functions instead of the real ones, and you might also encounter the problem of learned skills. If an INFP has a strong grip over his/her Si, yet prefers not to show his primary or secondary functions for whatever reason (social pressure, traumas etc.), you are likely to mistype him/her.
Some of you frequently type celebrities and people you've actually never met (!) at TypoC. Why? I don't see this as a moral issue, and I do realize that typing others might have some uses for both parties: they can be more tolerant, compassionate etc. towards each other, but what if you mistype someone, and start a conversation with false assumptions? What if you talk to an INFP as if he/she were an ISTP? This might really screw up the connection, and it might reinforce the unhealthy behavior of a stressful person.
I have a personal policy not to type anyone except for myself. Do you see the possible costs and benefits (for the lack of better words -> yuck, I hate these expressions) of typing someone? Is it still worth it? Why? Are there any other benefits of typing people?