I think this makes perfect sense:
The only people you should ever try to convert to your philosophies are those who agree with you.
I didn't make my point well. The letter-writer on Media Whores was criticising Nader for speaking to a group of conservatives. If their goal is to persuade people to be environmentally "wise", then Nader should be speaking to those who aren't in the Democrats' fold. They should be praising him for speaking to the "great unwashed" (i.e. conservatives).
The evangelist John Wesley spoke in bars and town halls, not primarily churches.