The last battle in Saving Private Ryan is another good example of FIBUA.
The US troops were entrenched in the BUA. They had MG and snipers up on the high floors.
But the majority of the troops remained on the ground. This will usually be the case as those on the high floors always run the risk of being trapped.
Also, on the high floor you can't react to changes in the battle situation. If for example enemy is nearly breaking through one part of the defence perimeter, it will be difficult for troops up in high floors to go and reinforce that area.
Saving Private Ryan also showed the value of tanks in FIBUA. When enemy is entrenched inside a concrete building, there's nothing like a good old tank canon to blast them out.
This awesome combat video of The Big Red One in Iraq gives you a good idea of FIBUA. A lot of times you are shooting at concrete as you can't see the enemy. It's another kind of jungle.
www.combatreform.com/ArmyBIGREDONEinfallujah.asfIt gives you an idea of how helpless you feel with a rifle or even MG for shooting at unseen enemy behind concrete and cement. You really hope for a big tank cannon. Otherwise you'll have to clear every room in that city.