User blog comment:B214/Question and Answers 6/@comment-99.235.66.221-20161130072057/@comment-10330822-20161213104216

He has to be strong enough to maintain the armor, which is easier said than done. Some people forget he has to be physically as strong and durable as Vali in order to use the armor to the same degree. We know this because Euclid demonstrated that the more powerful your base is, the more powerful you are with the different versions of the armor. For example, Euclid is Ultimate-Class/Satan-Class on his own, so when he wears Scale Mail he's strong enough to fight Issei with Cardinal Crimson Promotion.

This means that since Issei and Vali are roughly as strong as each other in their different versions, than they are pretty close to the same strength without them. Vali is just more balanced.