I like the way you guys think creatively about why Rivendell wouldn't have siege machines while other factions would. I think you guys are very thoughtful in how you've approached it from a story perspective. But I think there's still some justification even in the story for siege machines. The Dunedain of the North often retreat to Rivendell after long excursions defending much of Eriador. Surely there would be old forts and such that would have started to have been taken over by orcs or other intelligent evils, and certainly evil men descended from the days of Angmar, as Sauron was stirring up all kinds of evil things in the world again--especially if you incorporate the Hobbit films' lore in that Sauron was trying to cause the realm of Angmar to rise again. Whose to say Sauron never caused fortifications to be built in the ancient past using the One Ring, so that the foundations for those fortresses remained intact, and watches upon them would have needed to be made. Having largely failed in this watch over the millennia due to the various crises that more or less ruined civilization in the North-west of Middle-earth and vastly depopulated it, maybe the Dunedain eventually discovered to their horror that at the end of the Third Age some of these fortresses were being reused and rebuilt, and had to be brought down again? Whose to say places like even Carn Dum weren't refortified...? Fortifications like that in the mountains and hills would indeed be too well defended if these descendants of the Numenoreans couldn't build stronger weapons than battering rams!
Even Elrond expresses deep concern in the books about the growing power of evil in the wilds around Rivendell. We know so little about this evil. I think my approach would be a great way to open this up to further ideas to interpret the potentially advanced nature of it.
Perhaps keep the Revelers but otherwise remove their Akallabeth power to be replaced by siege machines?
I also want to address the gameplay style itself. I'll risk saying this at the cost you may have heard similar arguments before, but it's worth saying, regardless. Rivendell really does need more traditional siege weapons of some sort. When the enemy has a ton of towers and guarded structures to defend itself, like Lothlorien, especially, the combination of that enemy's archers and the guards that spawn from structures, to say nothing of garrisons stationed there, becomes too much for Rivendell to handle without siege engines. Plus, against certain factions, such as the Misty Mountains and even Lothlorien, the Akallabeth power doesn't seem to do much against their primary structures, fortresses, for some reason. Is it the armor of high level of these structures? I've had experiences where I've pummeled and pummeled against a fortress of the Misty Mountains faction with the Akallabeth power to very little avail. A bug? I'm not sure. And battering rams are too beginning game for such strong defenses, even while they may do very good damage in large numbers of them. But if they can't get through mid- to late-game defenses, what's the point? Granted, Revelers may have longer range for the Akallabeth power than siege machines, but what good is it if they consistently do so little damage against the most important structures like fortresses?
As for damaging walls, Rivendell is at a severe disadvantage, especially if the walls are well defended by towers and siege defenses. Unless I'm experiencing some sort of bug or something, Revelers don't seem to do meaningful damage to these walls. I was at one point taking on a player who was using Rohan who built himself a wall, so I tried to use the Revelers' Akallabeth power to take them down, but it took a highly inordinate amount of time, even using five of them--and that was just one section of the wall.
Another reason siege engines are so important is that the advanced siege of Rivendell, the Revelers, are so vital to increasing the resource output of Rivendell for most of the game, until the Reverence of Yavanna upgrade has been purchased. And even then, should the library be destroyed, you would need to pull that advanced siege back once again to help maintain proper resource production, and thus being back to square one with having to use beginning game siege, but now complicated by the game being late game.
All in all, while I like the idea of the Revelers as a much more unorthodox siege unit, complimenting the advanced nature of Rivendell as one of the last vestiges of the power of the Elder Days, due to the aforementioned issues, both in story concept and in gameplay style and value, not having siege engines is more often than not a bit too offsetting for such an advanced culture as Rivendell and the Dunedain. Even having some sort of option to use siege engines or Revelers in a sort of different sub-faction approach ala Erebor's two sub-factions would be more preferable. Maybe a "Culture of Elves" and a "Culture of the Dunedain" approach, to be researched at the Library?