Grumpy_Mike:
because there will be losses losses during rectify.
No, the other way round. If the transformer is rated at 6V RMS the rectifying that will give you the peak voltage 8.4V. That is what you will see.
I forgot about that. I don't mess with AC very much especially rectifiers. That's why I'm here to write the ouline and fill in most of the blanks, and the guy who gets the Really Big Bucks corrects my errors. LOL!!
If you have a transformer then there is no need for an opto coupler.
-I- don't trust -anything-. If I'm patching a $30 consumer grade widget into a system that if the widget fails might shut down my business for a month for repairs, then I want as much protection as is reasonably possible. The opto protects in both directions. Imagine a series of errors/failures from the Arduino power source side towards the system that 'tries' to blow up the interface transformer. Insurance is priceless.
The transformer actually scares me the most because it's the only part directly connected to the system. That should be as industrially graded and tested as possible - nunna that CCC Cheap Chinese Crap from ebay.