According to Michelin, 8" is the max recommended rim width for the OE tire. However, every other Brand of 215/45/17 on the market that i've seen specs a 8.5" as the max. In the tire stretch scene, this is not even considered a stretch. There is no danger of running this width on the OE tire.
With the safety question answered, the advantage of stretching the sidewall to max spec is that it produces less tire roll during hard cornering thus improving steering response. There is an added benefit of an increased contact patch because more of the shoulder is contacting the ground thus providing more grip.
This tire loading illustration demonstrates the same principle of a narrow rim width vs a wide rim width on the contact patch of a tire.
There are several reasons car manufacturers wouldn't max out rim width and they include but are no limited to cost savings, better fuel economy, weight reduction, and comfort. Cars made for performance tend trade off the above mentioned aspects for better handling and road feedback.
As for damage to the rim, there is slightly less cushioning from the sidewall when it's stretched so there is a slight increase in risk of a pot hole ding but the OE tire is a 215/45 which is less susceptible than a 215/40 or 215/35. Also, as you can see from my pics, the OE tire rim protector is quite large and still protects the rim from most curb rash.