I wouldn't trust web services even if they are easier to use. How do you know the server or the javascript on the website isn't out to get you?
With Bitmessage at least you can not rely on any one service except bitmessage itself. There are more angles for attack on a web service than a heavy p2p client on your computer.
I'm sure sooner or later bitmessage will be ported over to other systems.