I think we can safely divide the potentual innovators in electric car design into two categories:
1. Start-ups and lone individuals who lack any significant patent portfolio of their own
2. The small number of big auto-giants (General Motors, Ford, Chrysler)
The promise was simply not to "initiate" any patent lawsuits as long as people where acting in "good faith". This is effectively the offer of a patent non-aggression pact, if you don't sue us, we won't sue you.
Startups can tweak and reinvent the technology, but they then can't sue Tesla if they borrow their ideas back. The promise avoids the barrier to entry of hiring a patent laywer before you can even start tinkering.
The big auto-giants would probably seek a more formally written agreement, but as Tesla Motors was first to properly research high end electric cars, so they probably hold many core and fundamental patents. Big auto will probably be infringing on something in their own electric cars. And big auto could probably find some technicality of their own mountain of patients to stick on Tesla Motors.
A long patent lawsuit might be costly for big auto, but could push Tesla Motors out of business completely.