My players recently started a new campaign after progressing to the end of the heroic tier. They've gone from a well-tuned engine of destruction to a comedy of errors.
We've only played about 12 hours so far in the new campaign, but there's definitely a learning curve associated with new characters and new party synergies.
Also, as the previous party progressed, they tended to take feats and powers that played to each others' strengths. The cleric would emphasize CA-granting powers in order to help out the rogue; the fighter would emphasize movement restricting powers because everyone else was ranged; and so forth. With the new party, they haven't yet figured out what each others' strengths and weaknesses are. (Some of them haven't even figured out their own strengths and weaknesses yet.)
So I can see some merit in the argument.