Why would you bother to code background-repeat no repeat, it won't repeat of you don't put it in anyway, surely?!?!
I think its done because when your frame is large and it has a small picture which would repeat to fill as a background image, that has to be restricted using the "background-repeat" function.
It's not a nuisance, it's an opportunity. For when you want to put it in there but you only want it one time.
let me try : 'no repeat' seems not necessary. but may have something to do with other css codes in the same page. some codes on repeats before may inherits down to where you are. so to 'no repeat' it is save measure!
it starts from the top left and repeats vertically and horizontally to fill the page unless background-repeat is used to restrict this