Interesting idea, although I'm left wondering what's in it for the operator of this service? There doesn't appear to be any "about us" or information about what's happening with the data I pipe to this url.
WRT putting sensitive info through it... I completely agree, I wouldn't advise this - at the same time, I'd never advise sending any sensitive info over any email (without using GPG). The reason for suggesting custom content was, if I had several commands going that would send notifications, I wanted an easy way to identify which one completed (even just sending a number or simple word so I could idenity if). The idea of piping to it was simply an idea of how I could expand it and suite other people's use-cases.
Hope this all makes sense.. so no mass data gathering etc, and, as I say, I wouldn't advise putting any sensitive info through it anyway :)