Call me decadent, but I'd rather have convenience. The separate battery pack is something that you have to screw with every day. The battery going bad is a minor inconvenience maybe once every few years. It's not a GUARANTEE that it's cheaper to replace - a good 7.4v external pack is $50 to $75. I don't know that Dinotte would charge more than that to replace the internal pack if it went bad.

Also if their sales pitch is accurate, it's less likely to go bad - they say they're using mil spec cells and are intentionally underutilizing them to maximize cell life.