I have a Jolla (Sailfish) phone. I can not use the web client, because I need an Android, Windows or Blackberry phone. WhatsApp does however work fine on my phone (via Alien Dalvik).
This "web client" is not actually a web client - it's just an app for your phone that you can display on your computer monitor.
Sailfish itself is much better than anything I have ever used - it's clearly designed for touchscreens. For hackers it's awesome because it's actually a Linux, and you can access shell (as a root, too).
Android support is a bit buggy though - for example Skype sometimes breaks (can not send messages), but Spotify and WhatsApp have worked perfectly fine.
If you are interested in it, they dropped the price a while back (down to 250eur).
The interop with Android works great. Spotify, HN App, reddit is fun, YouTube, Google Chrome and Google Play Store(!) work without problems except for some crashes from time to time.
The only problematic app is WhatsApp because the notifications only go into the native Sailfish notification area when an Android app is in the foreground.
Many apps and copy&pasting between native/Android apps were fixed in the last Sailfish version released 3 weeks ago.
Having a very quick look, I see a websocket connection, which IE can do just fine.
The only thing I see that it can't is the desktop notification, which you still have some ability, in fact 'pinned' sites in IE let you have a number badge on the taskbar, which is quite good for a chat app, see Skype.
I suppose the benefits of supporting the remaining 5% against the costs of having to develop and mantain another client(s) have been discussed internally in Whatsapp.
"It would be nice" and "it would be commercially sound" are very different propostions.
Android and iOS have more than 95% market share of the smartphones market. Plenty of people access the web with feature phones. In fact, that's why Whatsapp has clients for Nokia S60 and S40.
Apparently it is, since Whatsapp does write clients for them. You can't tell me that writing a J2ME app from scratch is easier than adapting a web client to work on a somewhat more limited browser.
That statistic is meaningless. The percentage of what's app users using a particular platform is far more important, assuming their customers matter to them. There are a lot of Nokia users on What's App. As far as iOS, with iMessage, what's app is redundant unless you have a lot of friends using another platform.
If there's a legitimate security benefit from What's App, then perhaps that's compelling, but trusting a Facebook company with anything remotely related to privacy and security is a fool's errand. Zuck has yet to prove that privacy and security matter.
For secure messaging I generally use a courier and a message written on paper using a one-time pad. Slower but more secure. If it goes over a wire, the odds of it being actually secure are low, unless you handle the encryption and control the keys yourself.
Doesn't excuse the Chrome-only-ness, though.