Game Over and A Year of Frugal Gaming left a great comment I wanted to share (Von, I hope you don't mind me quoting you here):
Von said... Cool tray, no doubt about that, but I think there's a reason Warmachine players don't use them, and a consequence for you that you might not have thought about.
The sheer openness of Warmachine Mark II's formation rules open up a lot of tactical options (with lines of sight/fire, casualty removal, counter-attacks and so on), especially for high priority units with different actions and attacks available to them (like Trenchers and Long Gunners).
I worry that you're locking yourself into one rather limited formation, and thus locking yourself out of those options. I also worry about terrain, particularly buildings, although I assume you'll just abandon the tray there.
Of course, strength of play may not be as big a priority for you. This'll certainly speed up your play, and if you did a battlegroup and solos one as well, they'd be nice display/transport utilities.
I hadn't really thought about how the tray would lock me in. Recently, I have played several games with pHaley where I line up two full units of Long Gunners, 24 mini's, and shoot holes in the oncoming hordes. I thought it was a pain in the butt when I had to move them all up the first turn, thus the idea a tray was formed. I think I also just wanted to play with power tools =)
I like the imposed spacing so I don't get all my guys killed in one round of Khador AOE's. I also think I can leave the tray when needs be. I knew going into it that it would probably only help for turns 1 & 2. However, once they are in the tray, my tendency will be to leave them in that formation simply because I am too lazy to want to move them all. I pretty sure I would pull them off it when I needed to but it would have to be a conscious decision.
Another thing for me to consider is that the Long Gunners are at their best when they don't move. If they stand still, they get a +2 aiming bonus on either 1)their first of two shots at 12" each or 2) on a 24" shot. They also have the combined range attack ability. I might just start leaving them in their deployment zone starting positions. They won't get a shot the first turn but they have longer to shoot at the enemy. I guess it still depends on the scenario.
Anyway, Von, thank you for the comment. It has really given me a lot to think about. I still plan on finishing the tray and trying it out a couple of times. There is nothing like experience to show me if I made a movement tray or an interesting display board!
Thanks for reading!