I've just read Joe Frazier's "Box Like the Pros" book and he is a huge fan of sparring, both a an educational tool and a means to greater fitness. He, too, puts a lot on the speedbag (though not as much as Tommy Noel; Jimmy McClarnin was in that league as a proponent of the speed bag), and he sees the mitts as a tool to practice techniques etc.. to be used and perfected in sparring. His special favorite seems to be the heavy bag.
Can't say I agree that less sparring lengthens careers...back in the day when fighters sparred a lot more they also fought more fights over more years, JC Chavez being a more recent example of this same thing.