Sea Slug and the early missile-development sprint

Was Sea Slug useful for anything beyond starting fires?

Yes. It was useful initially, but rapid advances in missile technology and changing circumstances soon overtook it.

Sea Slug was useful when conceived and when it entered service. Rapid advances in missile technology and changing circumstances then overtook it. The same pressure affected guided weapons throughout the 1950s, 1960s, 1970s and 1980s. A navy that fell behind that pace could quickly find that a recently useful missile no longer met its needs.