Give me a single use case for Ripple which is not covered without Ripple.
Bitcoin also does not cover much (if any) new grounds - it just makes them smoother.
And finally Ripple has nothing to do with Bitcoin - what is the reason to discuss this legacy system in this forum ?
See the title of this thread:
"Ripple: A Distributed Exchange for Bitcoin"
Ripple offers a system to exchange IOUs for assets like Bitcoins or others with each other easily on an open, mostly unregulated market. This thread deals with the methods on how to do so and some misconceptions (as well as some well funded criticism). Ripple is much older than Bitcoin (one could argue that the original concept "Hawala" exists already VERY long) and the current incarnation of RippleLabs is not the first implementation of such a system. In fact, there still exists a Ripple classic client as far as I know.
You as well seem to neglect the trading aspect of money. Within Bitcoin, all you can get are other bitcoin, which stand for themselves (unless you consider IOUs issued as colored coins or mastercoins - though then you have IOUs again). With Ripple all you can get (well, nearly all, there's XRP - but imho you shouldn't "invest" in these) are things that represent something outside of the system. Instead of restriction (which makes sense for an asset that tries to be worth something) you have abundance (which makes sense for a system that can map anything and everything and where tokens actually already represent something).
While Bitcoin tries to be the ultimate fiat money (in the sense of "there shall be money", not in the sense of governmentally issued money) and hard asset, Ripple tries to be the ultimate market and transfer mechanism. These can, do and will go hand in hand - for example since Bitcoin transactions can (nearly) not be reversed, they are also one of the most traded asset IOUs in Ripple right now, as gateways are happy to receive them. It took Google just 2 days or so, to shut down the wallet account of a gateway that used them to buy XRP in contrast (it works again... now you can buy XRP which are easily convertable to BTC with Credit Cards again, probably by far the cheapest way to buy BTC with CCs). In the future, this will get even better and better, with people not even realizing that their Bitcoin payments were bought on the fly on Ripple. Adding in Litecoin support or any other non-reversible, pseudonymous currency/payment system for that matter is also just a matter of coding a small gateway.
Just like some people here boasted that "through bitbills, someone might have already shopped at your service in BTC without you even realizing it", Ripple already works this way in the BTC direction right now. I would say that alone makes it worthy of being allowed a thread here, other projects are unfortunately not that far yet.