A legend isn't required, but when approached correctly, it can be an effective sales tool. Before you offer your customers a drink with a story, think about whether you can make that drink on the more lively days of the week. If the answer is no, forget about the legend – it won't help you deal with customers' negative attitude when they don't get the advertised drink
In specific situations, a legend is just useless. Like when you're doing something crazy for no apparent reason. Serving a Negroni in an elephant, for example.