I do think that it is important, and that all applications meant for a broad user base should be written for all operating systems.
The problem with it is that the tools available for cross platform development all have something lacking.
Java is bloated to the point of not being usable, as well, since Java is, and always has been, a single platform by virtue of being a virtual machine, it really is not a cross platform language.
Perl, Python, PHP tend to be slow on Microsoft's operating systems.
PHP doesn't have a scripting environment on MS' products, only the web interface option.
Ruby isn't as widely known, so it's cross platform tool kit isn't tested as well as it needs to be for enterprise use.
objective-c, well, the only os that makes significant use is OSX.
C and C++, the biggest drawback is that there is no one toolset for them, on ALL operating systems. GCC is tweaked into djcpp for windows / dos, but those changes don't make it GCC on windows / dos natively. cygwin and mingw are making a linux environment on windows to run gcc in, which defeats the idea of one toolset native on all operating systems.
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