I believe the majority of people play the game wrong ever since the third class came out and specially now that you're able to get all 6 classes. Before these changes, most combos were pretty equal in terms of DPS so it didn't really matter what combo you played. Think Lv50 elites or lower.
Since then though, the gave went through several substantial changes. Instance design (burn or die), Lv60 and Lv70 elites, some reworks (Scout, Rogue/Mage), and new races with their respective classes really reshaped the way the game works.
Now, if you're a Mage and you want to burst DPS, get Warrior or Warden as a secondary. If you want sustained DPS get Scout as a secondary. Solo stuff? Mage/Priest or Mage/Druid for the self-sufficiency. This is the new class-uniqueness. You're able to do different jobs in a single toon.
That's what I've been doing. That's what people in my guild have been doing. You know who doesn't do it? People who pick a non-burst DPS combo and then complain that their class is not good at burst DPS.
What's the point of class combo uniqueness if all class combos burst the same, sustain the same, etc. Endgamers just pick the best tool (class combo) for the specific job at hand. That's, in my opinion, what people should be doing. To bridge the gap between other Mage/x and Mage/Warrior (2 30% dmg buffs, one buff that gives you around 80-100k INT, a 2nd Energy Influx) you would basically need to turn them into Mage/Warrior... lol. At that point just reroll to that class...
When I speak of class balance, I mean balance between the primary classes: Mage to Warrior to Scout to Rogue. There should be a Mage/x that is able to burst as good as a Warrior/x etc. The difference would come to the target. E.g. Boss is a humanoid so a Warrior/Warden does more damage to that certain boss e.g. (Pillars of Morfan). or the boss is some sort of caster so they have way more MDEF than PDEF so Physical DPS do better than Magic e.g. Mandara, boss is a tank so physical dps do less and magic blows it up etc.
That's my 2c on class balance.
Hypothetically speaking, say you're running IBP HM. If you were to take away one of your M/W's and replace him with another equally geared mage (m/p, m/k, m/r), would the overall DPS loss make the group unable to complete the instance?
Most likely yes. Other than Mage/Warden (which reminds me, I should actually test when I finish updating my gear), any other Mage combo would probably do half of a Mage/Warrior. IF THAT. You're going to be literally carrying someone through who SHOULDN'T be there. You can replace them with a Lv50 buffer who will do more for the party to guarantee success. That's really sad and pathetic.
One or two wipes can be the difference between 2 runs a night and only 1 run a night.
EDIT: To fix class combo gaps through elites you literally have to give, for example, Mage/Warrior NOTHING for a Lv80 elite and another combo a big buff. Might as well replace/rework old, useless elites for bad combos instead of having to come up with a "nothing skill" for the already OP classes...