"Those who say" to drive the s**t out of it are repeating an old wive's tale, urban legend - whatever you want to call it. The fact is that production engines require some time to wear in tight fitting parts, and avoiding this step can cause damage. The extent depends on the production tolerences of the parts, which due to the ganging of tolerences can cause galling or worse. Or you may never notice the damage if it is slight. There is no downside to following the recommended breakin procedure, but there ARE downsides to driving it too hard initially. No, wait, there is a downside - impatient drivers can't wait, not being familiar with delayed gratification. A sad character fault...

Expensive racing engines don't require an extensive break in because they are assemnbled to very tightly controlled tolerences, so time is not required for tight fitting parts to wear in together. Our "cheap" production engines are not so lucky.