it's not 48 hours of talk time, it's 48 hours of "average use." Regardless, your argument is that we only need enough battery to last 1 day, and not any more. Problem with that is how do you define 1 day of use (how long of a day, what kind of use)? How close to 0% battery life are you willing to get at the end of the day (i don't want the phone shutting off right before i put it on the charger)? Also, do you mean 1 day when the phone is new or 1 day when the phone is close to 2 years old and you're about to upgrade (because batteries degrade quite a bit after 1 year)?
See what i'm getting at? It's like the boyscouts, better to be prepared for various contingencies. I think that 2 days claimed battery of "average use" whatever that is, is probably the *minimum* you'd need to be prepared for just 1 long day (think 8 am for work till 2 am for partying), of frequent playing games, bluetooth, gps, navigation, towards the end of the phones lifecycle (when the battery has substantially degraded).
I mean, my gnex was enough for me for 1 day, most of the time. I don't think i ever saw 3 hours of screen time. I had to buy a new extended battery after 1 year. I had to always be keeping an eye on battery. Limiting full use of the phones features and towards the end of the day thinking about "can i afford the battery to do X?" and "do i really want to push it to 0% or save some juice for emergencies?"