Synergy is the idea that things work in unison, that they complement each other, where one 'group' of classes will overcome the weakness of another 'group' of classes, so that when they work together they become stronger overall.
In most traditional Trinity Games, the "healer" style class would be all but helpless in ability kill a mob and "melee" class would fill in that lacking functionality but be unable to heal themselves and thus when they work as a group they cover each others weaknesses and give direction to each others strengths.
IE: The "healer" keeps the "Melee" healed, the "Melee" keeps the mobs off the healer and kills them in process, and thus they continue through dungeon crawl reaping the rewards as a team.
The reality is, Synergy is about eliminating weakness and augmenting strengths of an interaction so that the unit as a whole becomes stronger.
In simple terms:
Synergy is when the Whole is more then the Sum of it parts.
In DDO, there is none of that. The Divine and Arcane classes in DDO are simply not designed with weakness that need to be filled by any other classes in this game.
So in DDO, What is really happening is that as you add people to the group you are simply throwing more raw DPS at the dungeon, you are not augmenting fusing class traits towards each other to augment the overall benefit of the group.
In simple Terms:
In DDO the Whole is Equal to (or less then) the Sum of it's parts.
However, in DDO, The classes synergize very well within a single build, and that is where the real joy and freedom of this game comes from. While I can't speak for anyone else in this game. but I enjoy trying to find new and more fun ways to build self sufficent characters, sure, we all could make cookie cutters, and what have you, but as you said, where is the fun in that, the fun lies in being able to build a self sufficent build, to push the boundries, and to see what kind of synergy you can find this game, and lets be real here, yelling 'heajls me" is not synergy in any sens of the word, when the guy with the Barb icon is is raising the main tank mid raid, while the Monk is raising the cleric, that.. my friend is a group that has found Synergy like no game has seen before.
Or is just such my observations. I return you to your petty bickering about how Divines are not Healers now.