"If i'd wanted a .30-30, I would have bought one"
Me thinks you are missing the point. Velocity of a given cartridge is determined by a number of factors; weight of bullet, ballistic coefficient of the bullet, amount of powder in the load, and length of barrel.
So the point is that if you shorten the barrel on a 308 it is still as good as or even better than other .30 cals which are considered good cartridges. And you can make up the difference in velocity by using a lighter .30 cal bullet and/or adding more powder (within limits), and/or using a different kind/shape of bullet.
Change any of those factors and you change the velocity. Since we are talking about the 308 we are talking about a .30 cal bullet which runs from 100 grs up to 220 grs. and can be used in a wide range of cartridges.
Ultimately, you could load a 308 to approximate anything from a .30 carbine load up to close to 30-06 velocities and everything in between.