Hi,
I love speed, but its overall value depends.
In a recent game, for example, I was evil (mmm, evil) and had my Psyonic Beams, medium hull and warp engines. I could have filled my ship with an overwhelming number of PBs, but saw that a single, expensive ship wouldn't serve me as well as fleets of less expensive, less offensive ships. IIRC, my level of miniaturization allowed me to fit 5 PBs and one engine on a ship. For nearly the same cost, I could build *two* ships, each with only 2 PBs but 4 engines. I needed the hit points, and preferred less expensive ships, because simultaneous fire was killing one of my ships in most significant battles. My fleets travelled at speed 15, allowing them to strike, retreat and reform. Or possibly reform and not retreat. Or do reconnaissance in force.
My ability to control the battlefield through maneuver meant that I almost always chose which battles to fight. Adding an extra combat bonus to speed seems gratuitous; it's valuable enough.
I might have made a different choice had I faced a different composition of AI fleets, or if I had better defensive techs.