Lewis, how would that be an advantage over a BOAB??
Not that I use mine now but I alway found that I could pull the applicator out of my bucket, laden with water, slap it in the BOAB and there would be enough water in there to do a complete house, didn't even need to take my bucket out of the car most the time.
Though I can see that it could be useful if you are doing a very big job, or perhaps doing lots of shops or other work whereby it isn't practical to get your vehicle close and you have to carry your ladder, bucket and other equipment some distance.
So perhaps some form of converted sack truck might be of use, somewhere to store all your equipment, carry poles, spare scrims, your water and so on.
Easy to lift out of your vehicle so you can just wander off and get on with your work.
Ian