User blog comment:B214/Volume 23 discussion blog/@comment-175.136.214.240-20171016073623/@comment-29963364-20171027015135

There are two reasons Issei went through the trouble of using DxD Mode (nerfed).

1.) To "Show off." Ravel suggested that since they're likely going to be facing more Satan-Class and God-Class teams during the tournament, it was best to try to "rattle them a bit" by showing off the overwhelming power of the DxD Mode (nerfed), and how much damage just one attack from it was capable of.

2.) To destroy a big number of "objects" in one fell swoop, without having to waste time looking for them (they managed to destroy 4, and got a few members of Baraqiel's team).

Also, the Pocket Dimension that the actual RG match takes place in is made sturdier based on the estimated strength of the teams and their members. Remember when Issei and Sairaorg fought? Their punches were sending tremors throughout the entire field, that the Pocket Dimension was breaking apart. That was because the RG Officials didn't know Sairaorg had the Longinus, Regulus Nemea, or that Issei would unlock Cardinal Crimson Promotion. They didn't make the Pocket Dimension sturdy enough to take the sheer power of their fight.

But, now everyone knows better to make the Dimensions for the Azazel Cup more durable, since God-Class beings will be fighting.

Oh. And, actually, Issei did kinda break the Field of the RG against Baraqiel's team. There was a huge Dimensional hole. Heck, Crom Cruach unleashed a huge attack to make a Dimensional hole just to show that he can do it too. So, Issei isn't weak. Dragon Blaster levels entire cities & briefly warps space-time, while Infinity Blaster levels entire mountains, and tears a hole through a Pocket Dimension made to withstand the power of Satan-Class beings.