Wouldn't having a vessel capable of high warp be useful to you?You mean a Runabout wasn't a shuttlecraft?Runabouts are intended to serve as an medium size vessel somewhere between shuttle & starship. The Danube class runabouts seen on Deep Space 9 are only capable of warp 5. They're bigger, capable of faster speed, and more generally built for long missions.
The DS9 producers created them as a way of getting the cast off of the station. That's the non-canon answer.