For some places, it's a large fence: the artists, designers, attorneys, etc. don't touch the perl code, and the developers do not affect the "look and feel" (e.g. the HTML). Of course, there are times when you need to communicate with the other group, especially for large changes, but in general in keeps everyone happy. It's kind of like having a subroutine in embedded in the HTML - the designers put a call to the subroutine in (say, to get a list of phone numbers) and the programmer promises to produce that list of phone numbers wherever that template is called.

Template systems at their best function as a contract - if they (the HTML designers) give this template, the developer will promise that it will get replaced by exactly *this* and *that*: no more, no less. It really frees the developer from a lot of finger pointing and blame. As long as their code honors their end of the contract, they cannot be blamed for anything regarding the look (and, if a good contract, even the functionality) of a site. So they can work on coding, instead of being blamed for a display problem that is finally linked to the lack of a closing TABLE tag by some designer. :)