That extra mobolity is only useful if you use a full DBB+turret setup which makes you utterly useless if you ever want to fight while still connected. If you go for full arrays like many cruiser captains do, you don't need the extra turn rate because it's easy to keep any target permanently in your broadside arc. I'd much rather have a big honking gun that I could use occassionally that acts as a free Lt. Cmdr tactical power.
Good points, except where you say 'only'. You're implying that the only purpose for speed and turn rate is to allow one to concentrate firepower into a tighter firing arc.
You don't need the extra turn rate, and you don't need the extra speed, but any cruiser captain worth his salt is going to know how to exploit both gifts to the fullest, and it's more than just about keeping the pointy end of the stick pointed at the bad guy.