In-class, allowing students to address professor informally ('tutoyer' / 'tutear')
Your question is completely subjective and context dependent. It adresses the balance between formal and informal, which depends on culture, personality traits, position in the academic hierarchy, etc. It's like asking, which are the pros and cons of teaching in full business suit vs Hawaiian shirt?

Some students will feel more confident if the class environment is informal. Some won't. Some won't see the difference. You (assuming you are the Professor) may feel at ease being talked in an informal tone, or maybe not. If you are in a new country or university and need to figure out your way, attend some classes and check which is the standard. Nonetheless, whatever you chose, don't withdraw your authority as Professor.