The idea is that dragons under the Berserk spell might try to attack several units at once. Plus, it's the only unit that can deal significant damage to Ents (it's probably not a good idea to cast Berserk on the Ents themselves, as their initiative is too low).
And, which is also important, dragons have very high speed and initiative; if they aren't affected by Berserk, they will cause a lot of problems.
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