Very useful post. And like you I came to the same conclusion, though I use hourly updates for the Fetch.
In my case, I am on a number of company email lists which receive messages every 5 minutes - killing my battery. I do have filters (including server side ones, set through Outlook Web Access over Internet Explorer on a Windows XP VM!) to move messages addressed to me or cc'd to me in to a specific folder. But the general flood of messages was killing the iPhone's battery.
My IT admin tells me that Outlook has specific push settings that allow users to set rules that say "push" specific emails. For example, you can set a rule to only push messages with to/cc set to your email address. I haven't found these settings yet. I am looking at Outlook Web Access over Internet Explorer on Windows XP VM for the same - but no luck yet.
This will enable you to filter out anything that doesn't meet certain criteria.