An idiomatic expression would be as useful as a chocolate teapot.
Someone or something that is of no practical use is about as useful as a chocolate teapot.
While the definition sounds quite harsh, it can be used in situations whereby something seems like it would be obviously useful in a situation, but due to a glaring error it's not.
As per the example in the phrase itself, the problem is that you need to make some tea. At first glance, a teapot would be useful, as it is the proper vessel for brewing a good tea. However, as it is made of chocolate, it serves no use in actually making tea.