Roll up reporting is not a standard feature in Google Analytics. However with a little extra coding, you can have stand alone reports for specific i.e. product dedicated websites, and a roll-up report to give a global overview.

Generally, this issue mostly effects enterprise clients. For example, companies with brand specific or product specific web sites targeted at particular markets. Because of this specific need it makes sense to have separate, stand alone Google Analytics accounts for each web site. That way, segmentation, referral analysis, e-commerce revenue (or lead generation) can be analyzed in detail.