Yes, to be clear here for posterity, Gmail’s default for users is to load images through it’s proxy, so they are only (ostensibly) fetched from the server once per URL. Creating unique URLs for any loaded resources allow someone to record that the message was opened, once. This is best practice in the e-mail industry, just as using cookies is best practice for the most reliable data in web analytics.
Personally, I have disabled auto-loading of images, but I know I’m an edge case – most leave the defaults.
Since I created this topic it shouldn’t be surprising that my opinion is that Discourse should track views wherever it can, and e-mail notifications are a great way to do that, especially if people are replying via e-mail. (Think of the absurdity of a topic with 42 replies and 0 views!)