> Honestly I don't understand why people get all flustered over email validation
Multiple reasons, and, yes, context is important.
Landing Page: You have one, and ONLY ONE, opportunity to capture a potential new customer's contact info. If they make a mistake entering their email and you didn't catch it you'll loose them forever. You can't send an email to let them know they entered two periods by mistake, can you? They are gone and you screwed-up.
Every single potential customer is sacred. Thou shalt not loose them by being careless.
Forum signup: In general terms, if someone is visiting a forum it probably means that they want to sign-up. In this case, it is OK to make them enter their address twice, make sure they match and send them a confirmation email. They'll probably try to log-on later on and discover something went wrong and re-register.
While I said "that's OK", I also think it is bad form not to at least do enough validation of all input data, including email, to catch innocent mistakes. I think people who are against email validation might have that position because they don't understand it or gat bitten by a crappy regex expression and that is that.
Now your forum sign-up user is angry because they have to enter all of their information again and go through the process one more time. Who knows, they might make a mistake once again. While I don't have any data to back this up I would venture to guess that the drop-off rate for making a visitor enter all of their data multiple times is significant.
Payment Confirmation: Must check as much as you can.
From my vantage point taking ANY action that might loose or annoy a visitor is simply --to be kind-- programming. There's no excuse for that in my book.