Tips for housekeeping are often most of my physical-cash outlay when I travel, because I pay for practically everything else by card. The percentage is even greater considering how often I make a small purchase using cash precisely so I can use the bills I get back for tips. If hotels provided a way to use my card for tips I might use it, though that would deny me the enjoyment I get from the exchange of notes that often occurs over the course of a week's stay.