I don't see why Kenny shouldn't emphasize the importance of that decision in the meat locker. It's a decision where failing to back him up will endanger his family in ways more direct and immediate than any other so far. Without thinking it may seem Kenny's just concerned about himself getting trapped in the meat locker with a zombie, but one has to remember his family has been abducted into the house and is probably in immediate danger, and Kenny'll be thinking to himself: "If I don't save them, who will?". He might even have been concerned for Lee's and Clementine's safety as well, considering I've sided with him most of the time.
In any event, failure to back him up immediately and without thinking would be betraying him in a moment of dire need, no matter how reasonably you could explain your decision to him afterwards. It's hard to rebuild trust from that, especially considering you wouldn't even be alive anymore without Kenny.
Kenny and Lilly are clearly two counterparts as characters. Let's see which one Lee and the rest of the group would be better off without:
Kenny will so far have saved you twice regardless your choices: once by giving you a lift off Hershel's farm (I presume you would otherwise have been forced to walk through the zombie-infested countryside without any food or water), and again at the drug store, which brings me to Lilly; she won't necessarily ever save you (in my playthrough she didn't) but instead her dad, who clearly wouldn't be in the group if she wasn't, either, has tried to kill Lee once.
Lilly and Kenny have constant disputes over the leadership of the group, and perhaps my only critique towards Kenny is that he's letting Lilly run things most of the time. Lilly's dad seems as though he ate as much as Kenny's whole family when there still was a lot of food available, and Lilly's rationing, though necessary in the situation they actually ended up in, would've been unnecessary if she and Larry hadn't been there in the first place. I also can't help but get the impression Lilly (due to her rationing) seems to be content with the group sitting on their arses slowly starving to death, while Kenny would at this point probably seek a new base for the group with more food supplies, or at least prepare expeditions to scavenge food from further away than the nearby forest.
And finally, at the dairy farm: Kenny is instantly suspicious and the only one in the group taking initiative to find out what's fishy with the farm (which, as we all know, turns out to be quite a lot), while Lilly calls him paranoid but is just as suspicious herself, and just wants to leave asap.This would actually have been a valid course of action considering what happened, but probably impossible to conduct due to the fate of Mark. Kenny on the other hand arouses Lee's suspicions as well, which eventually causes Lee to investigate Mark's missing from the dinner. In the meat locker, Kenny actively seeks a way out while Lilly is, once again (drugstore dejavu) busy with her useless father.
For these reasons, I will keep having Kenny's back whenever I can and get rid of Lilly at the first available opportunity. She's far too whiny to lead the group, has no special qualities and is just all-round unlikable.