I have often seen and heard musicians [on lots of instruments] whether piano, flute, violin etc.. use the slight lengthening or mildest slowing on certain notes to emphasise the tune in a more individual way, or 'Rubato'.. and I think it is a useful thing to do for more complex or faster pieces, particularly a good way to bring out the particular melody, or to really bring into focus part of a musical work. It can happen quite without thinking in a particularly conscious way, rather than mechanically going at one fixed tempo, to emphasise a particular bar or notes, by that slower fixing onto it, for just a momentary thought. And in slower pieces too, it can bring notice to a melody or piece, in a more lyrical way.