how would this be different from having multiple maximized winboxes, and clicking on the taskbar buttons (start menu bar)?
It's a simple convenience thing. Do you use tabs in your Web browser? One could argue that you could simple open every Web page in its own window, and switch between pages as well.
I personally use Winbox most frequently on Ubuntu with Wine. Sure I can right-click the Wine icon, and pick which router I want, but tabs are more visible. I also often move windows between monitors and work spaces depending if I'm referencing configuration notes in a spreadsheet or text file, chatting with a client on Skype, looking at the Winbox wiki, etc. At different times I like my windows at different areas on my workspaces. Moving one window can be quite a bit more convenient than moving many.
But in a Tabbed interface, nothing can be seen side by side. This doesn't solve any of your mentioned issues.
If you are trying to edit and monitor about 10 devices and more then you must open and minimize each one to discover which one that you want to edit
For those wanting side-by-side views, then there's always another option to open additional windows. Perhaps I'm reading into your comments a little, but it feels like maybe this request has you on the defensive a little. If several users say they'd find the feature useful based on the way they do their work, then does each individual need to justify why? Can you imagine a Web browser dev arguing with users about the worthlessness of tabbed browsing, because opening multiple windows works too? I'd respectfully ask you to consider the feature, even if you personally wouldn't find it useful.