A spare on the tongue can only help with weight distribution. Many trailers are already too light on tongue weight. With a dolly wheel, there should be no effort loading on the trailer hitch ball anyway.
Obviously, its location, location, location. If the spare conflicts with the dolly or winch handles its not positioned correctly. There is a way to mount one on every trailer - just takes some ingenuity.
A flat is a flat is a flat. If I have one I one I won't drive more than 50 feet! The one time I had one and drove 500 yards on the interstate I not only ruined the tire, but the wheel as well. Not my fault though. It took me that far to slow down......LOL.
Or, you can be like many, many others and go without a spare and hope for the best. My son did that (much to my chagrin) and when he had a flat, for the cost of the off-hours Sunday tow truck, new tire and wheel, new spare tire, etc,. he could have bought 4 new spare tires and mounts, and still gotten to his vacation spot on time...