You are right. There is no way to hide cells, columns, or rows in Numbers. I suppose it may be mandatory for some uses, however I haven't run across the need in my (admittedly simple) spreadsheets.
Whether Apple will add them. No telling. Apple does not typically ask for user input on new features, preferring to do their own research. They seem to get about as many things right (and wrong) as companies that 'listen' to thier customers, so who's to say what works better.
Hold in mind that Numbers was originally created as cheap and easy to use spreadsheet for personal use, not a professional number cruncher, and it's been further squished to fit easily on the iPad. It shows.
Perhaps one of the other office suites will meet your needs better. Here are some reviews, though you'll probably have to visit the developer sites (or maybe someone will chime in) to find out if they can hide cells.
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