I believe it's better to begin on the dirt, but that's not always the reality for many older riders starting out. My Dad was in his 60's and I was teaching him to ride, and no way he was going to do laps in the dirt.
I do agree about getting a bike that is manageable. I prefer big bikes (hence my preference for the GT bikes vs. the sport bikes) and some of them are a little unwieldy at slow speeds. My 'busa is not nearly as parking lot friendly as any of those 650-ish bikes, but it's a dream compared to any big touring boat. Also, even in stock form, a liter+ performance bike can go to warp speed so fast, it's often not only shocking to new riders, but can be difficult to control for someone not accustomed to going so fast, so quickly. When my bike was stock, it would easily loft the front wheel at over 100km/h with only a twist of the wrist. Not sure how a new rider would react to that. "Take it easy." Sure. Why buy a 190hp bike to ride around like you're on a moped?
Last summer I watch a new rider park his heavy cruiser bike nose-in on a severely sloped street angle spot, and it took him about a half hour to turn it around so he could ride out. Somethings are just learned through doing, so I suppose now he knows.
Insurance is a big concern. It's getting more and more difficult to insure motorcycles, and I can't imagine a new rider taking a policy on some open-class GT or liter+ sport bike. It's getting tough for those of us with 30 years of saddle time. (see my rant about my modded bike, not resolved yet...)