It sounds like you two are only just seeing this theme for the first time now, having not followed it through the version variations. The cabinet coming in from the left is just a layout styling from the beginning of time.. Cabinet on left and a game list or wheel on the right. The idea of animating the cabinet sliding in from the left is just an extension of that. That styling has been seen before in other layouts like the well known 'at the arcade' layout in the 'great theme collection'.
Here it is most probably a necessity with current AttractMode because each 'dynamic cabinet' is made up of four to five components. When this is drawn in view on-screen (possibly either with or without 'free navigation') there is a likely chance you will for an instance see-through one of the components. So, to avoid that the 'dynamic cabinet' is drawn off-screen first before sliding in as one unit into view.
The 'cabinet leaving' animation was a recent addition to add another piece of uniqueness to this theme and also to show off more of the 'dynamic cabinets' which is the purpose of this theme. I think seeing two 'dynamic cabinets' adds to the atmosphere.
The right side of the screen is already quite populated due to the cabinet taking up the left half of the screen. Normally, from an artistic preference I wouldn't have various assets layering over each other (as in cabinet moving under game list wheel and scroll bar, etc). I don't think it would look as appealing to have cabinets sliding in from the right underneath those assets. For example I've already added a blur to the background video to make it easier to see those assets. I don't feel the leaving animation is as intrusive though, especially when you consider that you won't see it until you end navigation.
Visually, I can't perceive how you could get three of these cabinets on screen while still having an image based list/wheel with any/many other assets (scroll bar, system logo, game count, game meta data, etc) in view at same time... And still having an atmospheric background at all. Having a three cabinet setup, and scrolling through them left and right is a nice idea but really would need a different overall look, everything in different dimensions and placements, smaller cabinets, etc.
From a technical stand-point I wouldn't even attempt that at the moment. Possibly AttractMode Plus or some future released modules might make that more feasible.
I'm sorry if what I've typed above doesn't explain it well.. I've tried to give you my thoughts and reasoning of my experience thus far. But the only way to understand is to try it first hand. I'm in the process of modulating the cabinets so in probably beta-v8 you will be able to start playing around with layout coding yourself and seeing what animation you can come up with.