Hi, a previous maid here. I am in school now but for 3 years was a housekeeper...
First, I was surprised to learn that most people didnt know about tipping maids. I grew up doing it, not a problem, just a surprise.
I have two things to say on the issue.
1) I got paid decently and by no means NEED tips. However, I started out making the same as your cashier, etc... The difference is that I scrub your pubes off from toilets, and pick up your bloody condoms, dildos, you name it. It can be a very personal and disgusting job. Keep in mind, I understand that I chose this line of work, but few jobs require the extraction of your body fluids and other things of such a personal nature and in my opinion deserves a little extra consideration than other "tipping jobs."
There are some hazards not common in other jobs nor compensated for in my pay. For example, I was taking out someone's trash and stuck myself with a needle, had to go get tested and was not only a hassle but scary.
2) People seem to have a nice snort when they hear of concept of tipping maids. I suppose I just don't understand why people will easily cough up 2 bucks to the person who washes their hair for 3 minutes, compared to a maid who spends around half an hour in the room, organizing stuffed animals on the bed not to mention to dirtier part of the work. I believe it has to do with the fact that because there is rarely face to face contact, people do not feel the pressure to give something extra.
I suppose I am saying that whether you believe in tipping or not- I think people should be consistent regardless of what someone is right in front of you or you never seeing them. A service is being done whether you see the person or not- and tipping is by no means necessary- but dont cop out if you feel they deserved a tip just because you might never see them.