Kabuto summoned Deidara during the fourth world war, but only used him to keep the Tsuchikage busy while he (Kabuto) kidnapped Yamato. He then unsummoned Deidara and has never summoned him since then.
Edo Tensei makes Deidara immortal, which allows Deidara to use the C0, a giant explosion that covers an area of 10 km diameter, several times. Kabuto could have used this to eliminate the enemy units more effectively, but why did he not do so?
You got your facts a bit wrong actually:
He was actually summoned later, as a part of the Special Ops Batallion (or whatever the name may be), along with Sasori and Shin. They fought against Kankurou and Sai.
In that battle, he was sealed (along with Sasori) inside Kankurou's Kuroari. This, along with the fact that Kabuto was still testing the summoned ninja (since he still allowed them to retain their personalities and fighting style), is probably the reason why he was not used more effectively.
See here for more details.