I believe that this can be very useful for the mainstream user.

It is however much much more useful for Google from a business perspective.

Lower openrates mean less eyeballs and revenue, which will in turn translate into more adwords and display placements. Obviously not directly, and probably not in most cases, but it should, even if it's only a few percent or less, translate into more revenue for Google. How much is anybody's guess.

If I were an E-Mail marketer or a big brand though, I'd be furious and try any other channels but Google's. In my eyes, it smells like Google using its market power to bully their own customers. They pulled a classic Facebook with this one ;)