Look, dude, web software is often part of a software as a service model. If I let you dig through my JS, you would get the service for free. In short: No.
I think web developers are over estimating the sophistication of their work :-)
You can't argue based on a 'network effect' website that has obvious lock-in.
Look at some non-social webapp. Say an online image editor. If the source was available, copy cats competitors could spring up and take away market share. There's no upside to releasing the source code, and obvious downside (Assuming you're in it to make money and not just spread love).
Image-editing algorithms are published in text-books. Graphics API for the web (flash, java, canvas, etc.) are published in API documentation. Nothing is secret. If someone copies your GPLed Free software and improves on it you can go and copy the improved version if you want it.
If a competitor is blindly copying you, you probably have nothing to worry about (should I elaborate?.) And if your competitive edge can be taken away with cut-and-paste, then you have ALLOT to worry about.
But allow me to add that the great majority of web startups are running on the value of a "social network". Remove the users and the service is worth nothing. You can't say that about shrink-wrap software; even if the great majority of the people don't use it, there is that one or two lucrative corporate or government contract to make it all worthwhile.
So in that light, shouldn't the users of web apps be entitled to something a little more than "free use"? Say, a little assurance that the platform they trust so much of their private lives is transparent, extensible, and should the company sink tomorrow, available for replication?
What happens when you strike it rich, sell the startup to a Mega Corp and move to Tahiti? The users will be at the mercy of greedy suits who will fill data warehouses with every click, persona and profile of your users, the very same people who made you rich.
If I had the front-end javascript source code to gmail or salesforce or basecamp, that would neither give me access to other user's data, nor give me free access. Proof: I do have access to their complete front-end source code, just minified. Having it more readable would just be convenient.
Minification / Obfuscation != Security. Don't put anything in your front end code that you would not want an adversary to read (true of the backend too, if perhaps less urgently).