Welcome to WebmasterWorld Guest from 126.96.36.199
Eg, your scripts are working, but every time they run, a hundred errors are written in the the access log...now imagine a 100 visitors a day, each hitting the script, thus generating 10,000 new lines in the error log.
Cascading errors can definitely bring system performance down.
Is that the kind of thing that you are worried about, or something very PHP, sablotron, and XML specific? :)