Why Not Templates?
Uniqueness and Originality
Obviously the templates are popular, that's what the providers claim on their websites too. It would be true, but in this case, your website will lose its uniqueness. How would your design stand out, If every second website your customer browses looks similar? How will you cut through the clutter, if you lack originality/uniqueness? A unique website is always judged by its layout. If the layout is common, it doesn't attract the visitors.
The templates are rigid, you can't do too much of customisation on a template. The options are limited, what best you can do is to change text or image. Not to mention, majority of the templates look more or less the same.