Using at least Joomla 2.5 and 3.1. Under the template manager options the setting "Preview Module Positions" must be set to "enabled" for previews to work. My question is, why isn't this feature always on? Is it a security risk to have it enabled on a live site? Does it slow the site down? Since the template manager is an administrator component, I wouldn't think it would be a problem, but I can't see any other reason to offer this option. Thanks!
The reason is to avoid 'information disclosure'. If that feature is enabled, it is pretty easy to identify the site being a Joomla! site. It does not have any other negative impact on your site.
There is no need to enable this module after you have launched your site.
This option is used while we are developing the site,
Its helps the developers to identify which position the module takes.
something like debug mode enable options, some templates will give some border around the module position too.
so my concern is after launch the site there is no need to enable that settings..
Hope it make sense..