Aside from prestige and stopping border raids, would conquering Dorne have any real strategic or economic value?

Quite a bit, but you have to weight that against the costs of conquering and occupying Dorne:

• Strategic: in addition to avoiding border raids, incorporating Dorne means having a much freer hand in the Stepstones (no more Dorne supporting the Free Cities against the mainland), eliminating the possibility of foreign enemies landing in an allied Dorne as prelude to an invasion of the mainland, and having total control of the western coast of the Narrow Sea. 
• Economic: while Dorne doesn’t have much arable soil, what it does have is arable in all seasons, which is handy. Moreover, Dorne has a lot of high-value export-friendly commodities, from citrus fruits to wine to silks. So while Dorne is overall less productive than many of the other Seven Kingdoms, its per-capita productivity is quite substantial.