DOD does not endorse using WD-40 on weapons, and as my Reserver career continued, it got more and more offended at the use. When last serving on Active duty as an MP on Ft. Benning, the smell alone would turn you down at the armory for an immediate cleaning - on your time.
I also worked 8 years as a commercial locksmith, no manufacturer used, recommended, or would warrant locks lubricated with it. The industry generally found that lock cylinders lubricated with spray WD-40 was the No. 1 reason for service calls due to inoperative cylinders.
Urban myth it's not - WD 40 isn't the best at anything but a water displacing agent. Those who use it are guilty of simply not reading the maintenance procedures for their equipment, or just being lazy. Pro's use the appropriate lubes, cleaners, and degreasers as specified.
I asked on a knife forum if anyone knew of a maker who specified WD 40 as a cleaner or lube. After 6 weeks we only found one, Leatherman.
At work at an auto parts store, the biggest use of WD 40 is to clean the counters. It doesn't get it all, but customers love the smell. After 24 hours it dissappates and the grime starts building up again. We'd use a better spray cleaner but WD is so cheap we don't care.
I've never found anything I used WD40 on that something wouldn't do a better job - except spraying out condensation in distributor caps. Precisely an application it was made for.
It's a signal of your level of competence - use at your own risk.
