As far as coins spent at merchants, I can't even think of a feasible way to deal with that... it's one thing if you make a large purchase, but what if you made many small purchases? Mountains of paperwork...
This is the crux of it. Current rules will never allow widespread adoption. Way too much hassle for the consumer, unless you are only using BTC to make large purchases every now and then. Which is absolutely not the goal...or at least not in my mind.