Never. My posts may be inconsistent because I keep mixing my point of view as an end user with that of the UI designer in me (and the two of them don't always agree), but my arguments never fail. It's me, after all. How could I be wrong?
The way I see it, we have progress because people identify problems and solve them. And we have progress because people are trying to solve the secrets of the world surrounding us.
There is a faint possibility of progress by just fooling around for no purpose (in fact, that's what nature does: evolution), but if we took this way for comuters, we'd still be using punchcards as a UI.
Unfortunately, it's not quite that simple. You know the game:
The market is driven by hypes and media attention rather than by reason. Consumers don't make choices. Managers do. And they choose every media's darling du jour for integration in their next devices. Letting the hype build up and not at least trying to speak some words of warning (like: "Do you know which functions you use on your browser? Did you check if Fennec offers these basic functions at all? Copy/Paste, anyone?") is grossly negligent.

All the people who are mesmerized now because they can slide the internet to the left an to the right... You need to ask them now if this is want they want a browser to do. Or if they'd rather want a possibility to copy a phone number over to their SIP clients, send a link via email, save the time tables for offline viewing in case they won't have 3G access later when they need them.... and so on.
If they answer: No, sliding left and right all day long is fun, and I've never copied anything from a browser to another applicaton anyway, then it's fine. Really. But for those who do need any of these functions (or others Fennec lacks, but MicroB offers) it might have been important that the topic was raised and they were reminded of their actual needs.

That's what party poopers like me are for.