Here are some thoughts to address the risks that you identified:
The feature should respect that tags site settings, including “minimum trust level to create tags” If that is higher than the user’s trust level, then the tag would just get dropped if it doesn’t exist (which could be due to a typo).
If the user’s trust level allows them to create tags, then the new tag would be created (which could be a misspelling).
Yes, the chance of these errors is higher without immediate feedback, but on the other hand, this is a pretty “advanced” feature anyway, so I wouldn’t expect a huge onslaught of tagging mayhem were it to be added.