Like most background graphics, it adds to storage space and retrieval time, at the same time as hindering the clear viewing of the data. Bad on top of bad.
And don't give me the "oh it looks pretty" argument, save that for birds and flowers, not for spreadsheets.
( Though a good spreadsheet should look pretty, because of the elegance and clarity of the data there :)
Of course design and white space are important too. My reply was not entirely serious, but I stand by every word there. The official RNB guiding principles of spreadsheet design to follow shortly.