#### Steps to reproduce the issue

Because mod_articles_category is missing some functions, i needed to create a copy of it with extended features. Steps:

1) Create a new module, with the intention to display articles filtered by category-id and tags.
2) Use this to get the article-list-model:
$articles = JModelLegacy::getInstance('Articles', 'ContentModel', array('ignore_request' => true));

3) Problem is: you cannot simply add a filter-state like this:
$filter_tag = $params->get('filter_tag', '');
if ($filter_tag) {
$articles->setState('filter.tag', (int)$filter_tag);
}

That is, because only the backend-model has this filter. The frontend-model does not. And i didn't found a solution in the documentations to get the instance of an admin-model.

Expected result

Frontend-Model should either provide the same methodes as the backend-model, or there needs to be a possibility to get the instance of a backend-model using JModelLegacy::getInstance
or anything else, that provides a filtering-method that can be accesses using the framework and not own mysql-queries.

Actual result

See description above.

System information (as much as possible)

Joomla 3.3.6 stable release.

Additional comments

\administrator\components\com_content\models\articles.php:
Admin - ContentModelArticles already uses filter_tag variables (See lines 56, 111, 297).

\components\com_content\models\articles.php -> Missing filter-functions for tags.