My honest answer is no! Why would you want to give Pure Planet your spare cash at 0% interest, when you could open a Nationwide account and get 5% interst. In fact, if I recommend you we could both get £100 as well!
If your query is about why if you increase your summer direct debit your winter one has to go up as well, I imagine Pure Planet finds it easier. - and therefore cheaper for their customers - to have one simple algorithm controlling direct debit rates rather than devising individual plans for every customer. I'm sure that's a slight over-simplification, but I think basically correct.
Bear in mind that if you are determined to lose money by giving it to Pure Planet to look after, you can always make a one-off payment.