The problem with Freespace is that the Harpoon only takes up 2 cargo space. In Freespace 1, more/less also missiles took up 4 spaces, such as the MX, the Interceptor, Hornet, and so forth. But suddenly the Harpoon is 1/2 the size of the other main anti-fighter missiles, and it's very effective, so basically most peopel will just use Harpoon (along with larger missiles like Tornadoes and Trebs).
Basically, if the Harpoon were 4 spaces like the Rockeye, more people might use the Rockeye, but as it is I think the missile is largely ineffective and ammunition tends to run dry pretty quickly. It's one, and only advantage (aside from no aspect lock) is its relatively long range at about 1.5 clicks