Using a template and no publishing
For all intents and purposes, I can see no way of distinguishing between a preprint that is not intended to be submitted and a preprint that just hasn't been submitted yet.  Thus, if a template is useful to you for formatting, by all means use it.

One caveat, however: some templates also include statements of copyright or material like a journal's logo that do not apply to a paper that has not yet been published.  Whether or not you intend to submit, I think it would be poor form to publish such things in a preprint to which they do not apply.