When it comes to aesthetic uses, sentimentality is usually a clear sign of a reactionary posture.
Rogders and Hart vs. Rodgers and Hammerstein
Leo McCarey vs. Joseph L. Mankiewicz
Sentimentality is invoked with regard to what has been lost and which must be recovered and preserved. The object itself is obscured: what is highlighted is emotional wave of sentimentality that is alleged to be a marker of value. Examination and critique are unwanted and discouraged.
The reactionary impulse invokes sentimentality to bolster its legitimacy and power. How could anyone object to Mom and apple pie?
Brian