A manual valve body by itself won't hurt anything on a DD as long as you're not abusing the transmission by holding in a low gear at high RPMs all the time or shift backward to lower gears before you should. Plenty of guys are running manual VBs on the street. They're fine if you know how to drive them right. They probably just don't want to take the risk of you knowing how to do it right and are just telling you "nope".
If you can learn to drive it like the stock auto would shift, you're fine. That isn't saying ANYTHING about what kind of converter you have (lock-up, shall speed, etc). I personally wouldn't want one for a DD and would just go with a stick though. Shuffling through the gears up and down w/o actually selecting what you want directly would get old. In my driving 3-5 and 5-3 are as common as me shifting from 1-2. You can't skip a gear on a manual VB unless I'm drastically mistaken.
Last edited by Maxwelhse; 07-10-2014 at 12:40 AM.