1. They provide fruit for years with little work (in comparison to annual vegetables).
2. They provide structural beauty to the landscape.
3. They can help you to make friends. How? Usually the fruit all ripens at once, so you have to give some of it away... if you give someone fruit from your garden, they'll probably want to be your friend... or at least I would want to be your friend.
It is not very difficult to grow fruit trees in small spaces. Most nurseries carry dwarf varieties. Dwarf varieties are simply the smaller versions of their larger selves. Nurseries are able to make dwarf varieties by grafting the fruiting stock onto a dwarf root stock... the small root stock is what keeps the tree in a compact form. Dwarf varieties are great for the home landscape because the fruit is easier to pick... little to no ladder-use necessary! Dwarf trees are also excellent candidates for espalier. Espalier is a technique used to train trees to grow in a 2 dimensional plane. Notice how the espaliered apple, below, makes a beautiful living fence, and takes up very little space. Perfect for the urban garden.
photo: Raintree nursery http://www.raintreenursery.com/catalog/producttype.cfm?producttype=Apples-Columnar
Recently I have seen columnar fruit trees, like the ones above, more and more in nurseries and catalogues. Columnar fruit trees are very small, and can even be grown in pots on a patio garden.
Here is a cool video explaining how to espalier a tree