It seems I had to visit m.google.com/sync before it took effect (or it just fixed itself today for me), but yes, now I have no reason to switch off Gmail! Thanks Google...
As for N8ter, I was under the impression that email and such is all handled by a Windows Live backend and hence you only sync once? Regardless, email syncing isn't going to be the one draining your battery.
Synching multiple ActiveSync accounts does drain battery (well, that's obvious, the extent is what's in question). Not only does it send keep-alive messages and routinely check, anyways, but it's also looking for Contact and Calendar information from Windows Live - not just email. There is no way to turn off some of that stuff in the Windows Live ActiveSync settings for Hotmail. When you set up you only get a Toggle for email. (yes, I know there are other dispirate places where you can try to disable things but that's not intuitive and the average user will probably not find it anyways).
When you add more ActiveSync accounts, it impacts battery life. ActiveSync PUSH is not anywhere near as efficient as BIS PUSH on i.e. a Blackberry where the server sends the data directly to the phone negating the need for the ActiveSync (or any) connection to be kept alive. They can do this because every Blackberry has a unique PIN. ActiveSync devices do not work that way. It's a Hybrid PUSH/PULL solution not a true PUSH email protocol like BIS/BES on Blackberries, hence the connection requirement. That's why Blackberries have never had ActiveSync support. It's a battery killer compared to BIS/BES. That's also why RIM forces a lot of apps to go through their NOC for PUSH Notifications, Messaging, etc.
I am not sure what Google uses in their Android GMail App but anytime I hooked up Hotmail to my Vibrant via ActiveSync I lost hours of battery life, literally. I had the same experience on Windows Mobile Phones as well (using Gmail ActiveSync in conjunction with WL4WM "PUSH" email for Hotmail).
Dunno what you're trying to get at with that Windows Live Backend thing. All mail is handled by the server backend. The only difference is the manner in which they sync to the device. That is what impacts the battery. One ActiveSync account is decent, but when you get into hooking up multiple accounts it can really impact your device uptime.
And everytime you open an ActiveSync mail client, it polls the server for emails, as well. That's why you see the "Email is up to date" message whenever you open the WP7 mail client. Almost every email client I've used has this behavior, except the few BB phones that I've owned.
Additionally, GMail message delivery is slow as ****, so I don't really care to use it as a primary account. Hotmail has more users so it can't be that their servers are overloaded (unless their infrastructure is inferior to Microsoft's). It just takes significantly longer for messages to arrive to GMail than to Hotmail. I see it all the time when I forward emails to myself from and CC both addresses. It was just as slow on an Android phone, as well.
I'll keep the account since I bought like 30 Android apps, though. Can't really throw away all that money