> Numbers is a good, platform-locked spreadsheet editor for simple uses?

Numbers is not just a spreadsheet editor, it’s a full-fledged spreadsheet creator. Its features are sufficient for all but the most demanding users.

> Dropbox+Excel seems like it would do the same thing

For me, it doesn’t. Just using Dropbox and my stock OS (and providing I don’t use Numbers), I can view Excel files on my phone and tablet but I cann’t create or edit them. I still need to use a third party app and Microsoft doesn’t have an appealing Excel offering for mobile platforms.

> even with more flexibility (including version control, document sharing)

Numbers has version control and document sharing.

> supporting even more platforms.

Numbers is available as native apps for iOS media players, phones, and tablets. There’s also a native app for OS X. For Windows, Linux, and every other platform that can run a modern web browser, there’s a very capable web app, available for free.

Excel is available as native app for Windows and OS X. For Windows Phone it has a very bare-bones app, the Android and iOS versions are a disgrace.

> I don't think you can ever consider spreadsheet software to be 'user friendly'.

Maybe you should try Numbers, especially on a mobile device, and then compare it to Excel.

I would, except that I don't use iOS.

Then again, I carry my air on me everywhere I go, so I'll never, ever be editing spreadsheets on my phone. Seems like an odd usecase to me, though.

I was mainly referring to Numbers on iPad, which I use much rather than Numbers on OS X. I do however occasionally use Numbers on iPhone, when I don’t have an iPad or notebook with me. It works fine for looking up information, make small edits, or do some quick calculations (although I mostly use Soulver or WolframAlpha for that).