User blog comment:B214/Volume 22 spoilers/@comment-27083298-20161112215056/@comment-10330822-20161114213754

Well, don't forget that all of these abilities are stacked. Cardinal Crimson Promotion is a stronger form of Scale Mail, and Diabolos Dragon/Pseudo Dragon Deitifaction is a stronger form of Cardinal Crimson Promotion.

So mastering these lower forms would still be good, because it increases the total amount of power he can bring out in his higher forms. In fact, that might be "why" he has such good control of DxD in this tournament, because he spent the last few months training his Cardinal Crimson Promotion.

Also, like I was saying, if Issei himself makes his normal body even stronger, his Scale Mail will be even stronger as well. Hence why Euclid, who was Ultimate-Class in his own right, was able to fight Issei with just a Scale Mail, despite Issei using Cardinal Crimson Promotion.

If Issei was a similar level to say Euclid or Sairaorg, in his normal form with armor, he'd be even stronger in his Scale Mail and higher forms. Imagine if he was as strong as his Cardinal Crimson Promotion form, in just his Scale Mail. That would mean he'd be even stronger in his actual CCP and DxD forms.