I have no problems with our formal (parents invited, no couples)
The idea of a formal is that you have finished school, and dressing up in formal attire and attending a formal occasion is symbolic of the fact you've grown up.
There's too much pressure on students with couples (some break up before the formal making things awkward, some couples hook up all over the place to show off, others dont have a date at all and feel isolated) and also without the parents it just becomes an opportunity for loud kids to be loud, which happens everyday of the week anyway.