Web designers use vendor prefixes in their CSS stylesheets to improve browser support. Many properties can be currently written with their prefixed versions, even if they aren't strictly needed. This can confuse people or make them write inefficient code. Lea Verou's tool "prefix-free" can alleviate this problem, allowing to write simple code that will be transparently converted to contain the prefixes needed by the browser. It can speed up the web development, but it won't change the fact that browsers still need to load these additional properties behind the scenes. As web designers, we will be happier, but our users will still have to wait longer. We should never sacrifice the outcome for the user (even if we might work with a client) in favor of our own convenience. If we aren't improving the actual value we deliver, we are doing something wrong.
Sharing code snippets of working demos requires that all vendor prefixes are already applied, and that will further contribute to their spreading around the web, even if our goal is to create cleaner stylesheets overall. Then it can become a maintenance problem if we can't easily distinguish which prefixes come from "prefix-free" and which from simple copy-paste (another reason to avoid it).