In reality, it takes many hours of intense training to reach the level of skill needed to rappel into real world tactical situations. But when was the last time you heard of any SWAT team doing a real world tactical rappel?
Real Rappels
I can think of only two actual incidents where rappelling was used to insert teams into hostile locations. The first was Operation Nimrod, the successful 1980 British SAS Iranian embassy hostage rescue in London. Two SAS commando teams stormed the embassy, killed the terrorists, and rescued the hostages.
During this operation, the rear SAS team rappelled down from the roof into the building. Unfortunately, one SAS operator broke a window and got hung up on his rope when his clothing/equipment tangled. To make matters worse, a fire had started and threatened to burn him alive. Fellow SAS saved him by cutting him down and he fell unceremoniously to the ground. If this could happen to the SAS, one of the best trained Special Operations teams in the world, it could certainly happen to a lesser trained team.
The second tactical rappel operation involved a SWAT drug raid on a heavily fortified building, where the element of surprise was impossible. SWAT determined that the best approach was by helicopter insertion onto the roof. The team made its approach either by rappelling or fast roping onto the roof, made its way from the top down (classic military tactic recommendation), and cleared the building without incident.