First You should try it... You said "It isnt a exploti" so tell me how You can call it? When You killed the boss and You abandon dungeon (just ESC+ENTER/No portal), You receive 100% exp and spend only 50% energy, You think it is ok? You can find that information at steam community :
"Abandoning dungeons
Wowowow, hold your horses, before you start being angry at me! What abandoning dungeons gives, you may ask. Let me tell you.
If you haven't noticed, when you enter a dungeon, you only use 50% of the energy. Wow, right? But when do you use the other part? When you complete a dungeon. Got it? You need to kill all monsters and then die to a boss. Then press abandon and voilia!
You will have your exp from monsters, but not only exp, you used only the half of energy! Bear in mind, that you need to play dungeons on the Manic difficulty."
It is a exploit or broken game system for me... They should repair or change it... When You don't know about it you are losing too much to people who knows about it. It is ok for you?
it's not a bug
you can run the instance and kill the boss you will get the boss drop and spend energy for it
or you can leave as you want and you would be using half the energy 
what make you think it's a bug ?
just because you still level 10 or something that's does not mean everyone using bugs to level and the server must have a wipe xD
also for the exp
leveling with quests is the best way
while leaving before the boss will mean less energy spent for the instance
and high level instances the exp from monsters is sh$t
following the quests will level you up faster
leaving before you kill the boss = leaving the boss quest (after level 30 all the instances have a side quest for the bosses with other quests in the same instance)
ofc there is in every level catch up to the next instance's level i run a manic instance twice (from the daily quests witch also better than running with normal quests) and i level up for the next instance
all my gears are +xp% for faster level (and i am slow by leveling btw
) i am level 39 right now
i did calculated the exp in another post in this forum >> see here
Fornt Line EP 2
True Strength (with EXP Boost)
with + 15% XP from Enemy stat
(ENERGY SPENT: without boss 6/with boss 12)
>---> ONE TEAM MEMBER AT THE SAME LEVEL <---<
EXP:
before enter
1,034,474 EXP
before boss
1,190,065 EXP
after boss
1,209,570 EXP
EXP Gain:
before enter
0 EXP
before boss
155,591 EXP
after boss
175,096 EXP
boss alone
19,505 EXP
>---> ALONE <---<
EXP:
before enter
1,209,570 EXP
before boss
1,362,720
after boss
1,378,187
EXP Gain:
before enter
0 EXP
before boss
153,150 EXP
after boss
168,617 EXP
boss alone
15,467 EXP
but what i didn't say that the manic quest that i can do instead of going without it would give me more that 200k exp 
so in above if you add the quest
without boss 168k exp
with the boss almost 350k 
The Conclusion :
level up with quests run all the instance with the boss /level up using the daily quests (the green once)
get the gears and sell them / farm for your gears / try to save energy in other ways because not running the boss won't be that good overall