[bluesmoon]

The point of progressive enhancement is specifically to not maintain two versions. You have just one version of your code for all user agents, and then you enhance it for agents that have better features.

Now worrying about having to build your templates twice -- once in your back end language, and then in JavaScript -- is a valid concern. Code duplication is never good. However, mustache is supported by many many languages (http://mustache.github.com/) which means that you really can build your templates just once and call out to very similar code to populate them with data.

Yes, you would need to run tests against the site with and without JavaScript (or CSS or XHR or any other feature that you were using to progressively enhance your page, but how far you go is really up to you), but if that's automated, it isn't much effort on your part.

I'll be the first to admit that it's fine to cheat in the code to give your users a better experience, but I don't think it's a good idea purely to take development short cuts or cut costs. If our roads were built that way (and in India they are), you'd end up with potholes every six months (and in India we do).