Yes it would, although that isn't a problem in RO these days. The M-10 was also open-topped, but crews would sometimes fit plates across the top anyway.
At any rate, while I think it's POSSIBLE to balance out the effect of, say, a Ferdinand/Elefant, or a KT or whatever other uber weapon you want to include, the things you'd have to do to balance it would make the map not very much fun.
Here's my thinking. In any situation where you have a single supremely powerful weapon or advantage, the side with that advantage needs corresponding disadvantages, or the other side needs a correspondingly powerful advantage.
This results in several possible (bad) scenarios:
1.) The map turns out NOT to be balanced, and one side really IS in god mode. This is, obviously, the worst scenario. For example, see "Orel Redux" or other similarly pointless fanboy wankfests.
2.) The side with the super-armor (or weapon or whatever) goes into the map with the expectation of pwnz0r1ng j00 a11, only to be disappointed by the fact that, no, the map really IS a balanced affair and the other side can fight back on equal terms. Typically, the requests (not from the realism people, necessarily, but rather from fanboys) are more about dreams of basically playing in "god mode." When it turns out they're not in "god mode" because the map is balanced, they get pissy about things, so it's better to just not bother trying to model it -- at least not when other stuff is needed more.
3.) The map is so delicately balanced that it becomes what I refer to as a "see-saw" map. Or, think of it as a "binary state" map -- you're either winning or you're losing. There's never a middle ground where things are undecided and team skill/tactics end up being the deciding factor. Maps like these basically revolve around a single aspect of the fight. IE: "Take out the KT and we win the map" or "Capture this point and we win the map." Once in a while that's fun, but generally speaking I find maps like this intensely boring. They involve, basically, one side or the other being incredibly frustrated at one point during the map. An example of this would be if the Ferdinand was in, it'd be basically indestructible without being taken out by, let's say, six satchel charges. Up until that point, Russian armor would be utterly annihilated and the map would be extremely frustrating for the Russians. Once the Ferdinand was taken out, it wouldn't respawn and the Germans would only have Panzer IIIs to fight the rest of the battle (obviously, to balance out the Ferdinand's effectiveness). So, once the Ferdinand is dead, the map becomes frustrating for the Germans.
Maps like that suck, in my opinion. They also are often incredibly one-dimensional because all of the focus revolves around the one side's incredible advantage and neutralizing that advantage. Then once the advantage is gone, to balance things out, the side who had it is basically left undefended. I used to see this alot in Forgotten Hope maps where one side would have a massive terrain advantage or the Germans would have some incredibly powerful armor that when killed or stolen would result in a guaranteed Allied victory. Like I said, maps like that are a real drag to play, in my opinion.
Regardless, while I'm not against having these things added, there are (to my view) far better choices like the Marder III that could be added to really flesh out the game in a way that DOESN'T require "see-saw" balance on maps. Likewise, I don't think we should be adding stuff that adds very little by way of different gameplay. Right now, tanking is pretty much a "drive, park, and shoot" affair. Only on the 88 v. 76 matchups does maneuvering really count for anything. Late war (and a fair bit of mid-war) armor would result in more of the same style of gameplay. On the other hand, early war fighting, due to the drastically different nature of tank combat, could get really interesting.
Personally, I'd LOVE to see a tank map with KV-1Ss and BT-7s going up against Pz IIIs and Pz-IV F1s. If we could flesh that out to include Marders, Pz IIs, T-26s (with the 45mm gun), etc. it would make for a matchup of radically different tank doctrines wherein success would depend far more on maneuvering, clever use of terrain, and combined arms, instead of simply "park and shoot" gameplay.