I did a little experiment where I sort of tested this idea.
My field is loaded with small
apple trees. There used to be an orchard in the higher property and we have a lot of 4 year old
volunteer trees that just shoot up all over the place. So on one half of the field I mowed around the trees and trimmed them to one trunk and a nice neat top. On the other side of the field I just let them grow. They have many small stems and a lot of tangled branches. After 2 years I can say a few things for certain.
Moles, voles and
mice easily girdle a small tree if there is only one stem. They go right around it and in a year the tree is dead. The critters have a tougher time getting to the main stem when it's surrounded by smaller stems and shoots. The smaller stems are more palatable to them so they leave the hard to reach main stem alone. If they do get to it, they usually can't get all the way around it to kill it.
Deer will usually only eat buds and tips of twigs that are within reach. By having a lot of little sucker branches within reach you can provide them the snack they want without having to reach. If they have to get on their back legs to pull branches, they will break limbs.
Finally,
apple borers will get at some of your trees no matter what you do. I've lost some trees with only one stem because the borer eats them inside out. With many stems you have a chance of some stems eventually out-growing the risk from borers.
I know it seems crazy but... the tree knows what it
should look like. It knows how to protect itself from all sorts of pests. Our trimming is weakening them for the sake of "neat and pretty".
No more trimming for me. I'm convinced.