In the Gospel, there were 153 fish in the nets pulled up by the Apostles after they had fished all night, catching nothing. When you include the three Hail Marys said before the 15 decades, the total is...153. While fascination with numbers can certainly cross over into the superstitious, they are not without meaning -- especially when mentioned under such circuмstances within Holy Writ.
In an age when very few pray five, much less fifteen, decades on a daily basis, does it even make practical sense to ADD five more decades? Forgetting the hubris involved in adding to that which was given by our Lady herself, does it make ANY sense to make this devotion "bigger" or "longer", and in an age that is decidedly addicted to external activity? In a word, NO.