It renders some web apps unusable. Try reorganizing your Netflix queue on the iPad. Netflix's JS doesn't scale well to a queue with hundreds of items. On a computer running a modern browser, this means a lag of a couple of seconds. But on the iPad, it can be s full minute.
You're being downvoted because the article is comparing iOS4 to Android 2.2. Both are operating systems. iOS4 is on both the iPad and the iPhone.
In fact, the iPad and iPhone 4 are very similar in hardware as well. The most significant difference is in RAM, where the iPhone has twice as much as the iPad.
Mentioning his or her experience on the iPad is completely valid.
iOS4 isn't on the iPad and won't be for several months.
That said, of course faster javascript is a benefit. Shaving even fractions of a second off web operations has huge benefits for users and results in much higher usage and happier customers.
When the performance bumps are imperceptible, it will be time to make a distinction. But on mobile devices that's still quite a ways out.
Not to mention that a faster, more efficient JS engine means less time pegging the CPU while loading and interacting with a webpage, which means longer battery life for your mobile device.