I will gladly do the work myself if it means not being stuck behind people chatting up the cashier or doing complicated coupon/return/exchange/gift card transactions. The value is in the consistency and predictability of time spent for someone who just wants a single bag of onions or a single T-shirt. If stores had no-BS lanes (more than just "X items or less" lanes) operated by human cashiers I would use that too, but I suppose we as a society consider it as impolite or bad service, so machine checkout it is.