My own opinion, which is somewhat supported by various sources, is that Zell is better described as glory rather than honor. It isn't about living up to some deeply held personal code -- it's about pounding your chest and saying "I'm bad!"
You abide by Zell because you're basically showing off. You can win these ritual duels and thus prove that you're worthy of passing on your genetic material. Reproductive success is tied to having success in battle according to the code. It's a bit like a peacock's feathers -- the guy with the best victories has the best plumage and gets to pass on his genes.
With Revival, there's a conflict within the Clans, particularly the more militant ones (I'm looking at you, Jags). On the one hand, they want to win the war. On the other hand, the entire premise for the Invasion is that the Inner Sphere is unworthy. Remember, Zell is not about treating the enemy honorably -- it isn't chivalry. Zell is about proving you are the best. If the IS is inferior, you should be able to beat them while handicapping yourself.
The Clans know that the Inner Sphere isn't going to follow Zell. Ideally they want to follow it anyway because it provides the justification for the war. It would be philosophical proof that the Clan way is better. "We fought with one hand tied behind our backs and we still won." For groups like the Jaguars (as well as the newcomers like the Nova Cats and Steel Vipers, who haven't really had a chance to adjust to the IS yet), that's really important to them. For them to abandon Zell, they basically have to think of you as Dezgra, utter dirtballs who just deserve extermination.
The Falcons and the Wolves have always been more pragmatic. Screw honor, they want to win. When push comes to shove, they're willing to do whatever it takes. In the last Aidan Pryde book, he even comments on how the Jade Falcon leadership has set aside bidding for the attack on Tukayyid. He thinks it's very unClan-like, and wonders if the Inner Sphere isn't corrupting the Clans themselves.