I find live tiles quite useful. They tell me if I have unused email, the weather, the time, the currency exchange rate, breaking news etc.
Besides, the springboard UI is for tablets where the expectation is someone runs one app at a time. If they switch away from an app it's to run another app. It is not comparable to a desktop where someone may have 20 windows open and therefore their mental processes and context are built around that. I have no major objection to the start screen in Windows for tablets but this isn't what the thread is about - it's for the desktop behaviour.
Ok, I'll try and take this one "objection" at a time...
In OS X, there are two ways to see these same things, without polluting the APPLICATION LAUNCHER with information that has NOTHING to do with Launching Apps:
1. You can use Dashboard Widgets (what you Windows-Saddled people call "Charms", I think). With Mission Control, the Dashboard is a "Space" (Desktop) that is a mere "swipe" away (and you can set Keyboard Shortcuts to the Dashboard, too). And Dashboard Widgets can do many, many things, and even better, the User can create their own Dashboard Widgets in HTML/CSS/Javascript (Dashcode), or by simply "snipping" parts of webpages (maybe that is possible with Charms, too? Afterall, OS X has had the ability for around 5 years now...)
2.OS X has a "Notification Center" Pane (again, only a "swipe" away, that shows you the weather, calendar events, etc. And it pops up Notification Bubbles (if you wish) to alert you to things you really want to "track"/wait for.
And on OS X, the Dock icon for Mail (like some other apps) has a little "bubble" that shows the number of unread mail messages. That bubble takes about 1/100th of the Screen space of even the smallest Tile in "Metro".
So, unless you are some sort of OCD person that just must CONSTANTLY know how your Stocks are doing (in which case, wouldn't you really be wanting a dedicated app?), or the Currency Exchange Rate (Yeah, right!), I see little use in cluttering an APP LAUNCHER (which is what the Start Menu is (or was), PERIOD) with that detritus. Again, it's something that "Demos" well, but isn't TRULY useful.
Next, you mentioned that Launchpad is more suited to Mobile applications, where only one app at a time is "Running" (I think you actually meant "Active", but anyway). Well, there's no arguing that "Launchpad" (Springboard's UI) was developed for iOS, and that iOS currently supports only "Fullscreen" Apps. BUT, Launchpad was aimed at the 90% of users (including most "Power Users") that, MOST of the time, really only run one app at a time (be honest).
And besides, since we are talking about the "Modern UI" (Metro) here, if there is a way to DISPLAY 20 overlapping Application Windows in that abominable interface, I SURE haven't stumbled upon it. I guess there is a way to "tile" up to 4 (far less than 20!) Windows; but the number of apps is LIMITED BY SCREEN RESOLUTION (FFS!!!!). Screens have to be at least 1024 X 768 to even Tile TWO apps, and to display FOUR apps, you have to have a fairly whopping 2560 X 1440 pixels. But I'll tell you what, "Discovering" that ability is as NON-intuitive as it could POSSIBLY be. I had to do some Research just now to even know it was POSSIBLE. So, that means that 99.99997% of Users will NEVER even know you can DO that in "Metro". EPIC FAIL!