The problem is that as soon as you tie the ability to send someone an email to your bank account you immediately put poor people at a grotesque disadvantage. I wouldn't presume to speak for pg, but I doubt any savvy investor would be happy passing up the opportunity to be the first to hear about the next Google/Uber/Facebook because the founder couldn't afford to send you an email. If you want to limit the number of emails people can send then you need to tie the ability to something else.

That's why Bill Gates' idea was so good - his plan was to make people's computers have to do useful work (say, Folding At Home or the distributed Seti search client) rather than merely transferring money around. Wealthy people wouldn't be at an advantage because the work unit would be time: a rich person with a fast laptop that can test 1,000,000 protein folds in 2 minutes has to do 1,000,000 tests to send their email while a poorer person whose hardware can only do 1,000 folds in 2 minutes only has to do 1,000 tests to send their email. Time is effectively the same for everyone. It's a beautiful solution. Far better than boring old money.