Most of the time, using a ready-made template is a good choice and provide you with great value and time-saving functionality. However, there are times when I curse the day I downloaded the glossy, shiny, all-bells-and-whistles template and installed it on my Joomla site.
When I'm developing sites for my own use, using a template from RocketTheme, Yootheme or another company is a great choice. I have the 100% creative control of the project, and can decide if a functionality is needed or just "nice to have".
However, if I'm developing a site for a client, the case might be another. The client may have wishes that at first sight can look easy to implement, but when I start adding new functionality to the template I sometimes experience odd results.