I like site templates myself, as they do save lots of time. I purchase mine from a particularly monstrous source. Aside from the professional designs, I like that their templates rarely sell more than six copies each, which ensures that my site would be quite unique. That is, it would be very difficult to stumble on a website using the same design, given the tens of millions of websites out there.
As for optimizing such templates for search engines, it really is no different than optimizing one built from scratch. My site 1usenet.com is ranked near the top ten on several very popular keywords for my industry. As far as I know, heavy graphic content does not penalize a website in search engines.
I have to agree that some templates require at least intermediate knowledge to modify. The ones I use (with flash and css) also require tools like FlashMX, Dreamweaver, and Photoshop to use effectively.