Some say that IOS and OS X should not merge because the platforms are very different and a merge would not offer an optimized experience on either. The other side says they should because Microsoft has and that lets developers write once and it can run anywhere. Both sides are wrong.
First, as to that "optimized experience". That's just software and hardware limitations. That's going to rapidly disappear. Honestly, right now most mobile apps are substandard junk that we only put up with because so is every other mobile app. As phones and tablets offer more power, that will change.
So they should be merged? No, because that "write once" is just nonsense. Try that and you end up with just what that "both a car and a truck" points out - junk on both platforms.
The truth is that yes, they will merge, but only when hardware power and software cleverness makes it possible to do it right. Then life will be beautiful.