Validation is a perfect example of when `<*>` is necessary, but I am sorry to say that it is rare that one wants such a thing for signals; the applicative instance you want is for the *validations* themselves.

In fact, the reason I haven't chimed in on this PR is that I have never had a use for this operator in signals. Doubtless it's a fine addition which makes sense, though. If we're talking about funny looking operators which RACSignal doesn't have, but which would be useful, the one I keep on needing is the one which scans forward by applying each element of a signal of kleisli arrows to the result of the previous one. (I've called this `scanWithStart:flattenReduce:` in my codebase, but whatever.)