With the TC (Tank Commander) unbuttoned, he's got a much better field of vision, and if the tank is stationary his binoculars are invaluable for spotting the hidden T-34 over in those trees. (Why tanks aren't equiped with telescopic sights is simply a game function- I don't know why!) As it stands, binoculars are much better for spotting than the main gun sight, and if the TC has to be exposed, so be it.
The comments regarding tanks in close proximity to enemy infantry and engineers are very, very true- unbuttoned, a TC or Driver is just asking for trouble, and buttoned up the tank is virtually blind to nearby ground-pounders. Best to keep to more open areas and fight the tank battle instead.
Most tanks today have excellent fields of vision, even buttoned up, but for our purposes there is no substitute for putting your eyes on the target or terrain. Cupolas are the next best thing, but still woefully inadequate- and surveying the battle field through the main gun sight is simply ludicrous unless you're in a REALLY good hide position and stationary for a while. Even then, binoculars will still give you a better look at what's downrange.
The advice for Drivers to stay buttoned up is good, as well- there's no reason to be out of the hatch unless you know for a fact you can't be hit (HINT: You NEVER know that for a fact- you tankers know what I'm talking about... CLANG!! BOOM!! "Where the $*&& did THAT come from??"
As for Bow Gunners, it would be nice to have the position modeled so that person could unbutton if desired, but as it stands it seems that seat is almost always empty anyway!
All in all, RO captures the essence of the whole buttoned/unbuttoned thing- the eternal trade-off between visibility and vulnerability.