We combine billing to reduce credit card fees, which makes sure that more of your money goes to the artists you support. Because of the way credit card fees are structured, this is especially important for artists who have a large number of fans who back them at a small amount.
Credit card companies charge both a percentage-based fee (2.9%) and a fixed fee ($0.30). 30 cents doesn't seem like a lot, but it can really add up. Imagine that you back 20 artists at $1/month per artist. With individual charges, $6.58 goes to the credit card companies, leaving only $13.42 of your original $20. With a single charge, the fees are only $0.88, leaving $19.12.