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It is about what you're used to. AFAIK Opera had this tab handling since they introduced tabs (in v6?), but there are a lot of Firefox users taking a look at Opera and they are immedietly frustrated by this. It's not necessary the tab handling that's annoying to me, but the developer's reaction to that request. Opera is a feature whore, yet they can't add this one simple feature people have been asking for ages (if you read Opera forums, you know what I mean).
It's not the mouse2 back/forward that got me attracted to Opera (although I do have this in Firefox through an extension), but the speed of it. It loads very fast, renders pages quickly and there's the instant back feature (Deer Park has this also, but it's not nearly as good as in Opera). It's also a lot faster at opening/closing tabs (because it doesn't wait for mouse3 to be released).
The advantages of the "other way" is that when closing a tab Opera will go to the last accessed tab whereas FF and IE7 will take you to the previous tab in the tab bar. There are some advantages in both of them (many times after closing a tab you want to the page you were at before, but also many times you want to just to the next tab). This can be achieved in Opera through gestures or a keyboard shortcut, but not with a mouse3 click on a tab (the way I preffer).