The extended battery lasts quite a bit longer for me too. And, honestly, I see no downside. It does not really increase the size of the unit. IMO, this is the battery Moto should have gone with to start.
Also, and I freely admit this may be totally in my mind, but the phone seems to run a bit less hot with the extended battery. I never liked the amount of heat the phone CPU area radiates but that problem seems far less significant since I went with the extended battery.
Anyway, I see it as a must have.