I'm pretty sure I actually spend less on Xbox Live than I would if everything were done with straight money transactions because it is virtually impossible to buy enough points to cover things you want to buy without having leftover unspent points, the existence of which (even if temporary) makes me feel wasteful.

eg. Ok, I would like to buy this indie game for XXX points, but I need to buy points in blocks of YYY dollars which means after I buy this I'll have 200 unused points sitting in my account for who knows how long... screw it, I didn't really want that indie game that bad anyway.

Granted this is just one view of the situation and I may be an anomaly, but for me the whole point<->money system causes me to think too much which pretty much kills off any potential for impulse purchases.

Straight up dollar values and a click-to-buy system as streamlined as Amazon one click is what gets my money.

YMMV.