Motorcycles come into their own when touring (IMO). You can tour on bicycle too, but it's a lot slower (i.e. takes up more vacation time to visit the same number of different places) and more effort (especially considering bringing luggage up and over mountains).

I don't have the time to tour from London to Croatia by bicycle, and there's no way I could consider it in any two-week break. I did it last summer by motorbike, and it was nothing to cross multiple mountain passes a day along the way.

I've done both a lot, and there's no way you can compare riding a bike to going 250mpg on a motorcycle.

Of course, like many awesomely fun things, it's quite dangerous, which is why I won't start riding again until I'm 55 or so.