Maybe someone with a pilot liscence can confirm this for me, but i suspect riding is closer to flying than driving. So you wouldnt want to learn how to fly in an F-22, you would start in a sescna (sp?).
I think there are two dangers to full size sportbikes (I dont recomend a modern 600 as a starter either. Early 90's 600s are one thing, modern ones are no less dangerous than a liter bike). One is the comfort lever. in my expirience, riders start to get comfy on their bikes after about 2 months. that doesnt mean they learned how to ride, but it does mean they arent scared anymore. So people start playing around. Twisting the throttle on something that will do a 0-60 in 2.5 sec in one gear can end up bad.
The unforgiving factor is a more powerfull argument. Modern sportbikes have very aggressive rake angles, little trail, short wheelbase, touchy brakes, and agressive tire profiles. All of these things make the bike more nimble, but sacrifice stability to do it.
Any modern 600 can flip over backwords from whacking the throttle open in 1st gear. Any modern bike will flip over forward if the rider gets on the front brake too hard (when he sees a small animal run out in front of him). Most have peaky power bands so the rider is likely to stall a bike 3-5 times before starting out, than frustrated, might just dump the clutch with abit too much power and flip it (happened to more people than I care to count).
Most bikes will lock the rear tire from just downshifting a gear. Engine braking alone is enough. New riders tend to be terrified of locked tires, and cant be expected to know how to handle a slide.
Than there are tankslappers coused by the bikes aggressive geometry. Its a phenomena that terrifies expiriences riders, since the bike will very quickly yank the bars from the riders hands and buck hard enough to couse the rider to loose their hold on the pegs too.
As for twins vs inlines, most twins have a wider more gentle powerbands. So while there is more power in the low end, it is easier to control, so a rider is less likely to sping the tire due to a sudden surge in acceleration that is not proportional to throttle change.
Than there is the expance factor. Everyone drops their first bike. EVERYONE. I've heard of modern supersports getting their frames bent from falling of the stand. they are that light.
But I dont think people necessarily need to start on a 250. In fact I dont recommend a 250 to most people unless they are really small. A 500, a large (750+) standart (not sport), an SV650 are all fine starter bikes.