Edit (now the answer on this question): The (former known as) “simple tagging” plugin seems to just add management features for tagging (like mass editing, etc.) plus allows tags for “pages” *). The all over look let’s me think, that the plugin doesn’t offer anything that’s not delivered by wp core.
*) Pages & Posts are only builtin definitions of “post types”, like categories & tags are builtin “taxonomies”. So you got a 99% chance that none of your data get’s lost even if you deactivate the plugin, as the data should stay in the postsmeta table.
Anyway, i’d suggest to look at the plugin code and/or ask the author about the possibility of data loss after deactivation or uninstall.