The use of WD40 on a chain is as big of a question as Oil is and just as controversial.
I personally would not use it but I don't give other people shit for using it. I know it was not engineered for this type thing and that other products on the market have been and do the job properly. You also don't need to lube the chain every time you ride but some people prefer to do it that way too.
If it makes one feel like they are getting the most out of their money by using a certain product and taking extra steps in their chain maintenance, then by all means let them do it.
Never Trade the Thrills of Living for the Security of Existence.