the fossil fuel, trees and all aspects are maybe more relevant in a general smart versus dumb phone discussion then in a multitasking discussion... tasks running quietly in the background are probably more efficient then if the tasks have always to stop and resume whenever they get the focus back.
but that's just my gut feeling (inactive tasks are more likely to be swapped out of RAM, thus time / energy needed to get the data & instructions back from storage aso.)
of course, if the active tasks use up all the available working memory, swapping will become a performance and thus efficiency issue, but the N9 has 1 GB of RAM, plenty to go for your average mobile apps.

either way, "smart" phones are energy sloth's
then again, that comes with the territory.
being on-line thus connecting to a cell tower all the time to check statuses and mail boxes uses a tremendous amount of energy
period.
if on top of that you want to post pictures with location, or simply give updates about your (exact) location (thus using GPS), well, you may not even make it through the day with one charge.

use a N9 simply to make or receive calls and maybe send an SMS every now & then & you are good to go several days with one charge, especially if you disable the "default" saving screen and make it a black slab ¦-)))

all in all, the energy consumption isn't caused by the hardware or even the OS, even though hardly any Linux (or BSD) based mobile will ever get close to the energy efficiency of Symbian but by the "usage", respectively the "hype" of always having to be on-line & "updated" & all that
and lets not even start about m$ crap.
lucky if you can do your daily shuttle with one charge >¦-)))))))))))))))))))))))

just turned off all networks & put saving screen to "black" on my N9
will see how long it goes, doing nothing
last few days i had it in Flight mode (thus without network but with the default clock displaying the time in HH:MM; uses about 6 mA in that state...