Tabbed browsing is [...] less flexible [than MDI], because there's no way to display two documents side-by-side. Imagine if Photoshop used tabbing between images: useless! (In fairness, the hybrid model used in Firefox, where you can open a new window or a new tab, is a really good balance. I just wish there was an easy way to "convert this tab into a window" or vice versa.)
That's not a fundamental limitation of tabbed MDI. Do you have a Mac? Open Adium and with a couple conversations. You can drag the tabs - not only to rearrange them within a window, but also to drag one out of one window into another and back. It's intuitive and even has nice eye candy. (IIRC gaim has this same feature, though it doesn't look as nice.) I'd post a trendy screencapture video if I knew how to do such things easily. Every now and then I try to do it in Safari or Firefox and am disappointed that it doesn't work.