Recently, I helped an associate with a Wordpress shopping cart. They were running into problems because of how the developers had coded the plugin. On the front end it looked fine, but it was so oddly designed on the backend that they couldn't manage it.
Unfortunately, many site owners run into similar problems.
They choose an extension because they are impressed by the way that it looks, but once it is installed, the interface is so complicated that they find themselves unable to make it work for them. That’s not a big deal if you plan to have an outside development company manage your site. But for many businesses, that isn’t a reasonable option.
When developing or choosing a Joomla extension, its appearance is only half of the picture. The other half – and arguably the more important – is the end user.