The account balance can be used for promotions such as weekly or holiday deals to buy games cheap. For me personally is important that the price better than the disk version - it's worth it. Also for game add-ons, music, movies can spend money on Mircosoft goals. Whether an offer is good or bad, that everyone must be able to decide and weigh up for themselves.
I myself prefer in any case this classical type of cash funding and am glad that it is still available.