User blog comment:IamJakuhoRaikoben/Chapter 471: Until the Battle is Done, Review./@comment-5303512-20160202131244/@comment-7418318-20160202212853

Becomeing more dragon-like (i.e. going into Dragon Force) greatly increases their strength and resilience, so why would going even further and turning into a full Dragon reverse that flow and make them weaker or more vulnerable?

I think the reason why the Dragons wanted to prevent dragonification is because they feared that their children would go mad like Acnologia and/or lose their place among all the Humans of the world if they transformed... but seeing how Acno was revealed to be able to switch back and forth that last one may no longer be true.