Numbers (and iWorks in general) makes a decent
full replacement for Excel (Microsoft Office), if your needs are not too exotic. So, yes, it's Apple's answer to Excel: with enough compatibility for those who have occasional need to transfer documents between the two.
For a companion app for Excel, then you are right, Documents to Go is the better answer. It has better integration with DropBox (for easier transfer between the computer and iPad), and tends to have fewer formatting problems when you transfer documents between Excel and the app multiple times.
Apple didn't get the question wrong. They just were not asking the same question as you. They wanted to replace Excel with a simpler, easier to use app for the average user. You want something that does Excel on your iPad.
Nothing wrong with that. That's why we have different software companies and apps, so no-one is stuck with one company's idea of what's the best solution.
For the OP: Here are a couple of reviews (by forum members) of the more popular Office compatible apps.
Compare iWorks, Office 2 HD, Documents to Go, QuickOffice by richsdams
http://www.ipadforums.net/new-member-introductions-site-assistance/22431-hi-all.html
Office Apps Comparison by RedCoil (details and examples, very long)
http://www.ipadforums.net/iwork-forum/36629-office-app-comparison.html