Usually, the middle name in English culture is a connection with a past ancestor. For example, the middle name of my brother and my father comes from my great-grandfather. My own middle name comes from my mother's maiden name. My nephew has a Greek middle name, in respect to his great-grandfather. That's usually how it works, but of course it all depends on the parents' choice. :) Normally, we only use our middle name for administration purposes. Or if we hate our first name. :D

I always use my middle name, as a matter of fact, I have hooked it onto my first name, giving the appearance that it was given that way at birth. I use my confirmation name as a middle name. Using just a first and a last name seems too plain to me and I am a contrarian anyhow, so go against the tide in everything.

We only use our middle names more for legal documents. Middle names aren't really important. Some don't have middle names and some have more than one. Some have a middle name named after a relative, usually a parent or grandparent. I hope this helps

To use it is very formal, unless you are filling out a form that requires it.

I can't speak for all English speakers, but only time I use my full middle name instead of my middle initial is if a form specifically asks for it. When I sign my name on important documents or on a credit card reciept, I use my middle initial. When I introduce myself to people I give just my first and last name. Some people may use both their first and middle names all the time. For example, my sister-in-law's name is Mary Jane and she uses both names.