When you get down to it, I think this comes down to a big conceputal difference between the modern ultraportable device (a smartphone or similarly speced out web tablet) and a general use device (a desktop down, to most netbooks (as they usually run laptop/desktop OSes).
General use devices, for the most part, have unlimited amounts of internet, unlimited power, unlimited storage, and copious memory which can be effectively extended to the nth degree by swapping things out to storage.
Ultraportable devices have unreliable (and slower, even 4g) internet, massively constrained and conserved power (which effects both how much data they want streaming at a time and how much memory swapping they want to be doing), limited memory, and sharply limited storage that's got a fixed lifetime and which therefore isn't really suitable for being used as extended memory.
Because of this, while current general use devices are designed with full invisible (to the app) always on multitasking, ultraportable devices aren't -- both the iphone app and android claim the ability to kill any app running on them for any reason at any time, and make it the app's problem to store its data and muster it later when it's reinstantiated.
This means that when you get down to it, apps designed for general use browsers just aren't going to be generally reliable on phones. Sure, with webkit running on the major phone OSes, they'll run, but they'll also randomly crash when the phone loses internet access, or lose state when the browser gets killed and restarted (or just yields the mahine voluntarily when swapped out, in the theory that if it's running on the browser you don't want it slowing down the machine when you're doing something else.
So while the browser has taken over the general use device, the app is still king on the phone.
Moreover, there are other reasons you really want an app on a phone, not an in-browser application for anything you want to use for persistent communication. Given the unreliability of network on something used for communication while you're moving around, you really want your ability to access data be independent from whether the phone can access the internet. If the phone has cached the important /most relevant bits of information (and for phone or a messaging system, doing this right is built into the protocol)you can read recent messages, send new ones, view and edit and save drafts, all without -caring- whether you're currently connected to the net; it will rectify things soon enough when things are back up.
Of course, some of these problems will change over time -- just as the slow computer revolution over the last 20 years pushed the appliation to the back burner on the desktop and made the browser king there. Phone storage will grow and get less power intensive, as will phone memory, and it will get harder and harder to find places your cell phone simply doesn't work. But for now and for the forseeable future, even if you favor the browser on desktops and laptops and netbooks, you're just better off with k9mail and gchat and the built in google chat client and a decent text editor on Android, and, I'd guess, the same is largely true on the Iphone side for more or less the same reasons.
tl, dr: Use the browser on netbooks and better, but use apps on phones.
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