If the locked item just happens to be gemmed/enchanted/reforged exactly the way it needs to be, then no, Mr. Robot will not pick the locked item, and will instead pick the unlocked item to modify.
And, the example I gave was very obvious on purpose. However, Mr. Robot uses math to determine situations that are in fact NOT obvious. Like when you have items that are close. For example, say you have plate bracers Mastery/Exp with a lot of mastery on them, no socket, no reforge, and an Exp enchant on it. I could end this example here, because the ambiguity of the item I described is the point I was trying to make.
Even if that Exp was reforged into +haste, well for a paladin that's still vague, but if the item it was replacing is a lower ilvl, it still could be better, even if it's off-spec. That's what Mr. Robot determines.
I guess the point I'm trying to make is Mr. Robot does in fact consider "is this item better than the item I can modify, even with the tank/dps/healer modifications?" I do also respect your point though. You had a question and you asked it! What more reason do you need? Not to mention, I'm sure there are people out there who have their own reasons as well; RP servers come to mind, or like you said, perhaps it's just easier to have totally different sets, and mixing them will throw them off their groove.
Like Rev said... it never even occurred to them while making the feature. And if it's requested enough, it should make it in.