A friend of mine is a professional guitar player and he has always had manicured long nails on his right hand, but never on the left. Depending on what style he's doing he'll use his nails for finger style or classical, but when playing blues or jazz he uses a pick but still uses the other fingers on the hand to pluck chords instead of always strumming. It's interesting to watch.
I'm mainly a bass player...and there's definitely a certain fingernail length that is too long. The fingernails on my right hand start to make a clicky sound. Not good.