Fair play, once an enemy has seen the MVAM in action once then the cat is out of the bag, but so what? The species in Star Trek have a fairly good understanding of the abilities and weaknesses of enemy craft but knowing the specs of an enemy ship doesn't guarantee victory. Well known federation vessels are attacked all the time by enemies who are familiar with their capabilities and still come out on top.
Also, just because a ship splits into three parts doesn't necessarily mean each is less well armed. From pictures of the separated segments (I don't know if these are canon, probably not) phaser strips can be seen on previously covered areas (the flat sections between the automated drone sections). This would suggest extra fire-power rather than less.
The act of splitting a ship into three isn't a tactic, it’s a tool simply allowing the user to vary his tactics, again, more options. The Prommie, when in one piece, is a pretty sturdy ship and I should imagine the 3 separate sections don't suddenly become less armoured because they aren't part of the whole any more. Nowhere is it written that the separate sections suffer from reduced fighting capacity simply because they were once part of a whole.
Regardless of cost, I would rather send a ship with 4 crew and added capabilities into battle than 3 single vessels with 50+ crew a piece. The most advanced armies in the world right now are going down this route. Why send people when you can send an automated system that doesn’t need all the systems a living being requires to function? Given that the 2 rear sections are piloted automatically doesn’t mean they would be any less dangerous (especially if piloted by dedicated combat holograms), it also gives them the option of a ‘kamikaze’ strike if things go really pear shaped. In the event of the main sections destruction then I imagine the drone sections would do exactly the same as their modern ancestors (Global Hawk etc) and carry on with their attack pattern and return home unmanned.
In a time where the dematerialisation and transportation of living beings is performed as part of every day life I struggle to see how a basic docking procedure would throw up too many problems for star fleet engineers. I think of the Prommie as 1 ship with 2 war drones attached to the rear rather than a single ship in the traditional sense.
A similar idea to the Prommie, but without the MVAM would be a Defiant class sized ship carrying the Federation equivalent of a few ‘series 5 long range tactical armour units’.
Either way, I think we will have to agree to disagree on this one. I see where you are coming from I just don’t think the negatives outweigh the positives. If you are right and the three smaller sections are weaker and the whole suffers from structural issues then fair enough, bad idea, but if not then I cant see too many drawbacks.