There are many good reasons to prune in summer, rather than winter... including the fact that the weather is more conducive to spending time with your fruit trees.
Louis Lorette was Curator and Professore of the Practical School of Agriculture at Wagnonville, France in the early 1900's... he did not prune in winter... he pruned in summer. He was a master of the art of fruit-growing and was an attentive observer of growth.
Lorette pruned to allow free passage of light, air and sun to penetrate the branches (especially those on the lower limbs of the tree). Lorette wanted to see pruned trees bear as freely as those in the fields... growing naturally... and he achieved that goal, admirably.
Georges Truffat, who wrote of Lorette's activities at the time, marveled,
I counted up to 100 clusters of flowers to the yard on the branches and almost every
cluster comprised from ten to thirteen flowers. What struck the skilled grower most, was to see the flowers spaced out along all the lateral branches...
I even saw
fruiting spurs on stems and trunks of the pear trees, quite close to the ground.
The marvelous flowering of the trees at Wagnonville in 1912 was seen by many... both amateurs and professionals.
Of course, Lorette's methods were criticised and it was pointed out that 1912 had been a particularly GOOD year for fruiting... but Monsour Tandart, as Director of the school, had been
able to take an important series of photos in 1904 that showed that year after year fruit trees pruned by Lorette's method were regularly and abundantly covered with flowers.
The Lorette system is a success, for already some are trying to imitate it and some gardeners describe it under the name of 'normal summer pruning' in order not to to give the name
of the man who first employed the system...
To explain, think of 2 similary sized trees of the same variety of apple. In the winter when all the nutrients are down in the ground, you prune ⅓ off of ONE tree.
In the spring, the soil warms, the nutrients in the roots rise up, but find only ⅔ of the tree! What happens to the excess sap? It forces upper buds to break and the
result is lots of "water sprouts" (the Medusa Effect). The tree is invigorated and vegetative wood is produced.
The other tree is left alone during the winter. In the spring the sap rises up and fills the whole tree without producing "water sprouts". In July and August, you remove ⅓ of the tree. In the fall, only the remaining ⅔ of the sap moves down into the roots. Next spring, the ⅔ sap rises up and fill the ⅔ remaining on the tree... again... no water sprouts!
As Georges Truffat said,
The only law of nature is variation, in accordance with surrounding conditions. Try the Lorette method for yourselves...
The nice thing about Lorette pruning is that it is such a pleasant time to be working in the garden and spending time with your fruit trees!
It's strange that neither Louis Lorette nor Georges Truffat are listed in Wikipedia.