Which is part of the reason I think people focus a little too much on screen-on time. It's not the only thing to take into consideration.
I have to somewhat disagree. I believe it's a pretty good indicator of battery life.
Firstly, it's usually the biggest battery hog on a phone (assuming you have no signal issues) so all usage habits being equal, I know that my Galaxy Nexus which gets 3hrs screen time will not last as long as my Nexus 7 which gets 5 or 6hrs screen time.
Secondly, I know that I can get about 3hrs max screen time on my Gnex. This is just what I've noticed from monitoring my battery constantly as I do all my devices. With my usage habits, 3hrs screen time on my phone equates to about 15hrs total before my phone dies. Now I know quite often when it gets to that 15hr mark, I might have quite a decent chunk of battery life left (like 30% or so) which might seem like this argument is doomed to fail. But what I know is that if I've got plenty of juice to spare come 15hrs, it just means I had a relatively "light" day where I didn't really look at my phone as much....maybe in the area of 1:40 or 2hrs.
My point is, having a rough idea how much screen time I get lets me estimate how long my battery is going to last. If I check it midday and see I've spent an hour and a half with the screen on, I'm pretty sure my battery will be in the red zone around 8 or 9pm so I better find a charger before I go out that night.
So just to clarify, it's not that I equate screen time directly to a certain number of total hours. Your usage habits are definitely different to mine. It's just an indication. If you said you get a "full day" out of Phone A with 8hrs screen time, then you also said you get a "full day" out of Phone B but with 2hrs screen time. I'm gonna take Phone A for sure.
What would be REALLY handy (at least for me) is to know how much screen time you had with a Galaxy Nexus compared to the Nexus 4, and to also know roughly how much juice you had left after your "day"