I think both of you are right. To each his own. I have a few applications that are algorithmically complicated and it is impossible (or very expensive) to implement multiple versions of the same thing on different platforms. Even though the core stuff is say 60% of the codebase (vs 40% in UI), say measuring in number of lines, the benefit of doing the 60% in a single productive platform is very very important. I'm talking about correctness, maintainability, consistency between different versions, etc. The remaining 40% UI code can be ugly, platform-dependent, inconsistent, etc, and it won't bother me as much. After all, an error in the presentation is far less lethal than an error in the core logic. In this case, a common platform is strongly favored.

That said, indeed many apps are not like that. Going forward though, I would conjecture that there will be more and more sophisticated apps.