Mhm, not really. Categories are a GREAT feature, and IMHO very needed. Payment mode is something else - I din't know which English name to use better here.
A category describes what the money was spent for. A payment mode tells you how this money was spent.
Use case:
You have 1000,- in cash, and know your bank account has 2000,-. So you want to know how much you still can afford to pay with your bank card, without looking at your bank account.
This is not an essential feature, but could be implemented easily by adding a field to the payment, right?
It should have a sane default (None?) - and could be ignored in statistics if all payments are "None".