Isn`t that statement "Bad associations spoil useful habits " just plain common sense ?
Yep. As someone on the first page of this topic pointed out, it's the equivalent of the saying "if you lie down with dogs, you'll wake up with fleas." There are probably a hundred other ways people make that point.
The WTS takes it further, of course. Any association that isn't a JW in good standing is a "bad association" who can spoil useful habits, such as going to all of the meetings, going door-to-door, annoying everyone around you by constantly preaching, shunning disfellowshipped family and friends, and so on. Hang around the wrong people long enough, and you'll lose yourself to Satan. Next thing you know, you've grown a beard!