It's the opposite actually.
Training shock troops would take 3 months (it's recruiting not training) and that's a long period for just 20/30 men. If you consider the slow replenish rates, they would still be the most cared for units on the battlefield and the player would keep them very safe in castles to replenish when close to 50% losses. Besides, when a unit reaches 50% it is not combat effective any more. We have to take replenishments from somewhere and the autodisbanded units are a good "excusation".
The final idea pushing me to the autodisband is the fact that the shock troops are killers. Faster to move, higher morale, much higher combat statistics and a few special powers. Without a "brake" they will become super units with experience, hence the autodisband PREVENTS this from happening.