I have read some different theories on ya'll (original spelling). It came about in the 18th century after 'thou' started fading out. That is also when other variations of 'you all' started.
There are a few different terms it could have been a variation of. One being 'ye aw' which was used by Scottish and Irish immigrants. During that time it was probably considered good grammar. I don't care if it is considered good grammar now or not.
It's a harmless word.