Well there is a time and place for everything!
Not wearing a shirt and tie does not equate to being scruffy and cr*p instructing!
There are obviously some folk out there who find the attire a nice comfy blanket - customers and instructors alike!
One thing is for sure: s&t is unlikely to upset anyone but may make for a formal and stale atmosphere.
Whether that is always the most conducive to having a good time and learning a lot is open to debate.
MHO of course.
FD