The main point behind the idea is that it should be an easily replaceable component... 24 to 48 hours of service. No dry dock needed, just swap it out.
One could say that this part of the ship's technology was known to be in a state of flux when the ship was originally designed and constructed. And to make sure it didn't limit the life span of the vessel, this part was left as modular as possible. I mean, when you think about it the ship had the cone shaped structure while it was still in the design phase, it was (most likely) constructed with a larger dish (that may have been less efficient), which was replaced with a smaller one and finally given an integrated structure like shown for Phase II or in TMP.
Perhaps these earlier sensors/deflectors off-put a lot more radiation than later models? Will your sensor boxes still also have sensors in them?
Might have even been something as simple as putting out more heat while in extended use... the deflector burnt out the ship's power systems in
Mudd's Women, and it (along with the phaser banks and running at high warp) contributed to crippling the main engines beyond repair (while in space) in
The Paradise Syndrome, so it seems like an intense piece of equipment even without knowing it's inner workings.
As for the box structures... in the beginning (when they were still featureless) I wouldn't think they served any purpose other than to hold the equipment in place and provide the needed connection to the rest of the ship. Later when more features were added (for the series version of the ship) it would seem the addition of sensors would be the best explanation for those changes.
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A little off topic (unless you realize that my main motivation for doing these types of plans is to one day build models based on them) I've been reviewing my two starship models. I had wanted to build a model of the Enterprise based on my 33 inch plans, but realized that I would face fewer obstacles if I settled on a two-thirds scale version so I could utilize existing parts and decals for the AMT 22 inch cut-away model.
I bought two kits to use as donors for the project, and went through and figured out what I could use (or modify) and what I would need to basically build from scratch. I kept a modified secondary hull, altered the warp nacelles (including extending the inner trenches back further) and will use a modified version of the supports (because the nacelles are supposed to be closer together than the kit has them). The dorsal and primary hull are basically being built from scratch based on a scaled down version of my plans.
Strangely enough, this left me with enough parts to build a second (basically stock) 22 inch model... which I decided would be the Constellation. While closer to being finished, it is far from being done.
In the images above the model has been test assembled... I haven't glued most of it together yet (I believe a well built model can stand on it's own without glue). I've started painting the primary and secondary hulls, and thats about it.
Part of the reason for pulling out these models is to compare and contrast the dorsal I built for my 22 inch Enterprise with what I'm seeing while studying the 11 foot model.