Vitamins found in the leaves can be higher than the vitamins found in the fruit. This includes vitamin C, potassium, and vitamin A precursor. Potassium and vitamin C are not stored well by the body, so have to be replenished periodically by the diet. There is some evidence that dehydrating the leaves concentrates the vitamins, as long as the dehydrating is performed without excessive heat. Can we say valuable post-apocalyptic trading commodity? Also a great reason to enjoy herbal tisanes and justify the cost of an inexpensive solar dehydrator for the backyard.
Citrus greening could become a problem, but not if you are growing the plants specifically for the leaves. Greening is the newest, baddest threat to Florida's citrus industry, basically an uncontrollable bacteria that causes fruit to improperly ripen. However, leaves are unaffected, and the bacteria is not a human pathogen. The best time to harvest leaves is right before the lemon trees flower and fruit, as that is when they will have the maximum nutrition. Meyer lemons flower in December, usually, and fruit a few months later. The flowers are fragrant and nutritious too! Citrus trees are perennials, so care must be taken to sustainably harvest them.
As much as IFAS and the news sensationalize the plights of the citrus industry, most citrus trees are largely resistant to pests. Perhaps it is because those statistics come from the monocultured orchard citrus industry. Perhaps it's because most are already grafted before you have the opportunity to purchase the trees. Perhaps it is because most people's backyard citrus are not planted so densely so as to encourage the growth of pests and the lack of beneficial insects and bacteria. Remember this previous article about how the state destroyed thousands of homeowner's citrus trees, then lost a lawsuit because it was scientifically unfounded? It just shows that they don't really know the answers either.
The fruit has many culinary uses. It is used in canning to lower pH and prevent oxidation. It can be used to cook raw fish without heat, but dehydrates the proteins - changing the texture. Lemonade. Household cleaner. Fragrance. Color. Limitless possibilities.
It may have some medicinal properties. Although not thoroughly researched here in the US, lemon is used in Ayurveda in India. According to this study, lemon is used to treat disorders of the throat and persistent catarrh (inflammation of the mucous membranes usually in the airway, causing an overproduction of phlegm and mucus). The low pH of lemon juice inhibits bacterial growth. Certainly vitamin C is absolutely necessary in our diet. Remember this post where I wrote about vitamin C and guinea pigs (as a survival protein source)? We all need good nutrition.
The recommendations for lemon culture from IFAS state that lemons are more vigorous than most citrus species and that dense plantings encourage pests. They are recommended for the warmest, sunniest part of the landscape that does not flood. IFAS doesn't recommend propagation by seed, but rather grafting onto a rootstock specifically chosen to avoid certain soil pathogens.