But how about the tactical operator whose armor prohibits him from reaching a belt-mounted pistol? Or the undercover detective who mounts his holster to a MOLLE carrier so armor and weapon can be quickly donned at the same time, or carries concealed in his waistband? We don't all wear Class A unis with a polished set of duty gear during our shifts. The location of your holster rarely changes the level of retention you'll want, but it may very well dictate the type of holster you choose and the manner in which you draw it.
The key here is to understand the needs of the mission at hand. Once you've got that down, train with your gear in that configuration. How many of us have known a patrol officer who was transferred to an assignment that requires a cross-draw, vest-mounted holster, but the officer never trained with that gear? In case you were wondering, I have my hand up.
As a trainer, I see a liability for the department and a disservice to that officer. As a fellow officer, I worry my friend may be at a great disadvantage during a gunfight. You can't expect to be immediately proficient with that new system any more than you can expect to play the piano after your first lesson.
Remember, when we train with our holsters or any gear for that matter, we are teaching our minds to perform a conscious task at a subconscious level. The first time we draw our pistols, we have to think our way through the process. The hand moves toward the gun, finds the gun, gets a grip on the gun, works through the various retention devices, and finally removes the pistol and brings it up to the eyes. After several thousand draws, we simply make the decision to draw our pistol and the "muscle memory" we've built into our subconscious minds does the rest. This eliminates the need for conscious thought (something that is extremely difficult to do in the middle of a fight) and makes the process faster and much more efficient.
If you spend years training with a duty belt, you'll likely be looking for your pistol on your hip the first few times you try to draw that weapon under stress. If you move the gun, you need to let your brain know where it went.