Interesting. In your observation how good has the Company been at not hiring these so-called "Brilliant Jerks"? I've found one of the common failures in engineering management is knowing when to get rid of smart but disruptive talent.
No mention is made of why they feel the need to offer severance when they're shedding an underperformer, but maybe they want to make it a less negative event. Maybe the strategy is to try and avoid distracting or upsetting the high performers by having underachievers leave on good terms (this could explain the "goodbye" emails).