During the early phases of fire attack operations, getting water on the fire as quick as possible tends to "make things better." In many locales, fire departments use nurse tanker operations to support the initial fire attack while waiting on additional resources to arrive...and then a decision is made regarding expansion to a dump tank operation.
The folks most successful with nurse tanker operations tend to use some type of control valve on the supply line feeding the attack pumper and the siamese tends to be the most useful device. However, lack of a siamese doesn't mean a nurse tanker operation can't happen. A gated appliance can be used...its use just requires human intervention each time a hose line is connected or disconnected. During our March 2018 drill in McCurtain County, Oklahoma, the folks wanted to try nurse tanker operations and they only had a gated-wye. There was no problem, the operation proceeded just fine and water flow was never interrupted. A couple of adaptors were needed as was a person to open and close the valve, but other than that, the operation was a success.