When opting for the plugin solution over the functions.php option, or comparing the use of a plugin with a custom hack in the theme, you need to consider the following things:
- Is the plugin really needed?
- Does the plugin extend WordPress functionality in a way that is either too complicated or so stand-alone that it really isn’t an option to try and do the same thing in the theme files?
- Does the plugin need to work outside of the chosen theme?
A positive answer to all these means that you should go ahead with your plugin. If you can’t provide anything additional, you should really think things through again.