Shaft drive is great... but it weights a tone compared to chains.
And there's no new mid range bikes with shaft drive coming out, your looking at 1200cc bikes to get the benefit of shaft drive then you'll still be paying more than a chain set every 15k miles with the extra fuel.
It's an interesting balancing act... I reckon the difference between running at 40mpg and 50mpg over 15k miles is about £500. But on the other hand, chain lube and chain cleaner isn't cheap... that all adds up over 15k miles. Say you buy a new can of chain lube and chain cleaner every 2000 miles at £15 for both, that's over £100... then what's a new chain and sprocket set fitted? £200? More?
I'm sure running the shaft driven 1200cc bike does still work out as being a bit more expensive, but the convenience of never having to worry about chain maintenance would I'm sure soften the financial blow. As would the extra power from the 1200cc engine!
Of course, this argument falls apart completely when you consider purchase price... new Triumph Tiger Explorer is like £12,000, whereas the V-Strom 650 was somewhere between £6,000 and £7,000. Hence why I have one.
Also, I know that if something does go wrong with a bike's shaft drive, that's often not exactly a cheap repair job!
But if I could afford a 1200cc shaft driven bike, I'm pretty sure that weighing it all up, it would make sense for me to have one. All depends on your priorities though.