Cuirass and Greaves meging together means the game renders faster, meaning they can have more NPC's on screen/more armour types without slowing the game down a ton, i don't see how that's a crappy excuse. I can see what people are talking about when they mention customization but in the big picture its only one armour slot.
This isn't a defense of multiple slots (I have no opinion.), but I fail to see how merging them together allows the game to render faster and allow more NPCs to be on the screen or that other thing you mentioned. That second one is actually just impossible. The limit on armor types in a game is called disk/hard drive space and merging greaves and cuirasses really won't do anything to help with that (plus more variety in items have absolutely no correlation to game/rendering speed). As for the first one, it still makes too little sense unless Bethesda is that terrible at optimizing game engines. There would be something seriously wrong with the engine if an extra armor slot had such a drastic impact, if any, on performance. What I do believe a possible reason may be are more unique and aesthetically-pleasing armor sets. They could do that, anyway, but if you were still able to mix and match armor sets as unique as I'm implying, you end up with clipping... and critics really hate clipping so apparently an option needs to be taken out to annoy those who don't care about the clipping simply because having the option to do something flawed is, apparently, worse than just not having the option but otherwise having the same results.
So, I think this is more for aesthetic differences. For example, they can now make more creative midsections on armor, but the merging has no relation to armor variety (just not possible) and the performance thing just seems unlikely and more of an excuse, to be honest.