I know this is a hotly debated topic and I'm not wishing to open a can of worms. I was asked recently why we have the no cross-tier policy so I would simply like to share my perspective.
I think many of the players that have had their cities since the dawn of time have forgotten how very challenging resources can be when you first start out. Lower level cities use a whole lot more T1 & T2 compared to T3. Since we are encouraged to build efficient cities we build our boosts, and we mostly rely on our team members to help out with the unboosted resources. So if you have a team of mostly smaller cities, the overall demand of T1 and T2 is also way in excess of the demand on T3. If players are offering their T3 and asking for T1 who will then supply the much needed unboosted T1?
As a leader I feel not only responsible for having a balanced, efficient city, but also need to ensure that holistically my team is also balanced and efficient. In larger cities and fellowships where players have gangsta resources they can afford to be flexible with cross tiers trades. But while players are still building their cities and trying to figure out the correct balance, promoting cross trades is actually doing them a disservice. How can they figure out they don't have enough of a certain tier factory if you are supporting the lack?
Now i'm aware of the strategy of higher ranking players saving space by only building T3... my question is: where does your T1 & T2 come from? Someone is using time and resources to make it, right? Again, the high ranking players with plentiful resources may not feel the burn, but smaller players sure do!
I feel if you care about the well-being of your team limiting the amount of cross tier trades will be your concern. If you have a large portion of smaller players its important to adjust your thinking to accommodate their needs. If you are in a fellowship you don't play without having a knock-on effect on others, and (IMO) you are either helping or hindering your team mates.
Thank you for listening to my Ted's Talk