hmm well
looking at them as pure city attack it starts off 4 + 100% = 8 str.
unpromoted that's better than a swordmans 6 +10% = 6.6
The way the mods work though, each city attack promotion just adds to the base mod.
so city attack 1 adding +20% would be
trebs 4+120% = 8.8
sword 6+30% = 7.8
city attack 2 adding +25% would be
trebs 4+145%= 9.8
sword 6+55%= 9.3
cityattack 3 adding +30% (ignoring gunpowder)
trebs 4+175%= 11.0
sword 6+85%= 11.1
so effectively a treb is better than a sword for city attacking striagt from the barracks, barely getting beaten out at cr3 vs the sword pure city attack str.
Add to all that the major additional bonuses of trebs...
a whopping 25% retreat rate and the super sexy collateral damage... and well it's dual use as a bombard unit as well as a city attack unit and you have a winner that holds it's head high vs other dedicated city attackers.
Mitigating for the sword is that a trebs cost is 60 compared to a sword's 40 and well it's tech places it more on par with macemen than iron age swords. Also aggressive civs don;t get a free combat 1 promotion on siege.
In essense, exp'ed swordsmen led off by new trebs is a win win situation for both units as they compliment each other very well. Don;t fear using one or two to lead off an attack as they have a much better chance to live any collateral damage saves cr3 swords' lives.
Cheers!
-Liq