If you intend to use the service across a variety of different platforms, why wouldn't a web service be enough? I'm having a hard time figuring out a good reas

If you intend to use the service across a variety of different platforms, why wouldn't a web service be enough? I'm having a hard time figuring out a good reason to justify writing client software for even 'the big three' platforms, much less one for your PowerPC Mac. And I'd think that, honestly, having it be a web service is the answer to that need. I understand that saving to Dropbox gets you access to the same data everywhere, but if you already have access to the same data everywhere, without having to involve Dropbox (which not everybody has), why would you want the added complication?