Ad-hoc templates (portal mode sites only) are non-reusable templates intended for temporary use. Having templates that are not reusable can cause duplication of efforts, clutter, and possibly performance concerns. Generally, ad-hoc templates should be reviewed and either moved to a regular page template or deleted.
Besides being non-reusable, ad-hoc templates have another limitation in that they are directly tied to the page they are created on. That means that, if that page gets deleted, so does the ad-hoc template. For these reasons best practice is to make all templates permanent resuable templates. This applies to even one-off pages who's templates will never be reused such as your home page or 404 page.