That is not true because you can remove a coin from the slab, send it back and get a different grade completely. There are several users that routinely send coins and get regraded to get a higher grade. If you take it out of its case it is no longer protected against the elements.
You're having a very hard time convincing me since I can't believe a $10 coin (serpcoin DIY2 for example) is now worth $50-60 simply because it was sent in for grading.
The above quote you basically say:
1. Remove the coin from slab and get regraded for different grade than initially received
2. Doing #1 will get you a better grade (most likely due to perceived experience)
3. Doing #1 is dangerous and not recommended due to the elements (no elements in my work space luckily!)
You didn't pose any argument for why someone would not want to remove the coin from the slab which is what I believe you were aiming for.
I think it depends on the grade, nevertheless I am under the assumption that graded bitcoin pennys are in the range of $30-$35,,, would you agree? They are originally $5? if you buy them single right? but more like 1-2 if you buy in bulk.