I'm not the OP, but I can give you a reason. I support all our mobile devices at work so I'm always using different phones. (Using an WP 822 and a Blackberry Z10 right now.) Because they all use MicroSIMs I can just pop my SIM out of Device A and put it in Device B.
With the iPhone 5 using nanoSIMs I could no longer swap SIMs around. If I got a nanoSIM and moved my number to it, it breaks the old SIM so if I wanted to move back to another device I'd have to burn through another SIM.
But that's just my case. Most people are not going to swap so much so if you have an iPhone 5 now, just get a new SIM and down the road if you ever want to go back to the iPhone, you will have to get a new SIM. I personally wouldn't mess with adapters but to each their own.