Keshiks will, relatively speaking, actually get promotions more slowly with the Honor tree, and never anything like "nearly twice as fast" as a Camel Archer. Keshiks get 3 XP per attack, 4 with Military Tradition. Camel Archers get 2 XP per attack, 3 with Military Tradition. At best, Keshiks get promotions 50% faster; if you've taken Honor, that's only 33% faster. Do the math: starting with 15 XP, each unit needs 85 more to get to Logistics. 22 attacks by a Keshik; 29 by a Camel Archer. Seven attacks is seven turns: one tech at Standard speed. They'll level up at practically the same rate.
Sorry for the misinformation, I haven't played CiV in a while in anticipation of BNW and it sure felt nearly twice as fast the last time I played as Mongolia. I also didn't think about Arabia with Military Tradition, frankly. You must admit Arabia with Honor is an odd choice; viable but odd. With Mongolia, Honor is almost a requirement.
The point however is Keshik
s and in that respect, ironically enough, their biggest asset now that I think about it is their relatively low ranged strength. Ranged cavalry are powerful due to their ability to attack from afar and their ability to avoid return fire with their speed, neither of which pertain whatsoever to actual attack strength. Putting the subject of what makes ranged cavalry powerful aside, an army of Keshiks will likely gain Range, and Logistics much earlier than Arabia. A turn of tech can be a large thing in a game such as CiV, but putting that aside one, Keshik getting a vital promotion one tech early is one thing but an army of Keshiks getting a vital promotion one tech early is a game-changing thing. Ironically enough, the weak strength of Keshiks aid in this acquisition of promotions by generally not killing everything on sight (why the Mongolians have the weaker ranged cavalry, I am not sure).
For plinking an Industrial/Modern city to death with Keshiks/Camel Archers strength doesn't matter as much as their promotions. An army of Camel Archers is powerful but an army of Keshiks would likely gain the same strength but faster, however.
The simplest answer to this however would be an MP match given equal assets with an equal army of Keshiks vs Camel Archers. I don't have much experience with multiplayer,given my generally inconsistent internet, but knowing what happens in a level playing field against two real players would likely show which of our views is more viable in actual play. I generally found my CiV perspective on early snowballing bonuses (the Keshik reaches its apex faster compared to the Camel Archer) while you seem to prefer staying power (Camel Archers stay viable for longer). Both, in my opinion, are valid but seeing an actual match would be the best way to sort this. A long, very large and bloody match against two players with two of the best UU's in CiV for title of best UU in CiV.
Also, it would be either amazing or boring to watch. Maybe both. I can't possibly be the first person to have thought of this, right?