I get email, but I am not using the gmail app. I like HTC's email app, because, well...just because
I do challenge anyone's stance that the setting does not affect gtalk though. With the setting off, gtalk is a crapshoot. With it on, its as reliable as BB Messenger.
Based on your post I decided to give the HTC Mail app a shot. All I can say is, thank you. Much better interface, mail is now integrated with People, saving attachments is easier. It even has the "Conversations" view. Fantastic.
I set mail to check every 5 minutes and went back and unchecked the "Enable Always On Mobile Data". Emails are coming right through. Thanks again. Anyone who hasn't tried the Mail app needs to check it out.