Mendelssohn, who always liked a "nice, swift tempo," repeatedly expressed his aversion to Chopin's rubato.
rubato is necessary in emotional music and is an excellent means of picturing longing, persuading, dreaming, et cetera.
In it he would no doubt have given many valuable hints regarding the correct use of the rubato.
It might be said that this dramatic rubato is something different from Chopin's rubato.
Those who have entered into the spirit of Chopins works will easily see when to use the rubato.