Just read this today - thought some of you might find it interesting. It's from Principles of Genetics (6th Edn.) by Tamarin; I've only edited the formatting a little for clarity:
In DNA, nucleotides are connected across phosphates. In 1987, F.H. Westheimer asked the question, "Why did nature choose phosphates?" The answer seems simple enough. The phosphate molecule has several properties that make it ideal for linking subunits into polymers in the biochemical world.
First, the phosphate group can form linking bonds - it can thus connect two compounds.
The linking bonds that are formed from phosphates (phosphodiester bonds) have the additional property of being stable yet are easily undone by enzymatic hydrolysis. In other words, the bonds are stable, but the nucleotide residues can be removed to conserve them during, for example, various replication and repair processes that we will discuss later. When a nucleotide is removed, the nucleotide is not broken down in the process.
After the phosphodiester bond is formed, one oxygen atom of the phosphate group is still negatively ionized. This property makes it less likely that the phosphodiester bond will be broken spontaneously (a negative charge protects against a nucleophilic attack), and negative ionization keeps the nucleotides and the DNA within membranes, specifically the nuclear membrane of eukaryotes and the cell membrane of prokaryotes. Since negatively charged compounds are extremely insoluble in lipids, phosphates are kept within membranes by this ionization.
Westheimer concludes: "All of these conditions are met by phosphoric acid, and no alternative is obvious."