Perhaps in response to the accusations of self-justification against Hughes, Wagner quotes Seamus Heaney's verdict on Plath's poetry. In his lecture "The Indefatigable Hoof-taps," Heaney explained what he saw as her limitation:
There is nothing poetically flawed about Plath's work. What may finally limit it is its dominant theme of self-discovery and self-definition, even though this concern must be understood as a valiantly unremitting campaign against the black hole of depression and suicide. I do not suggest that the self is not the proper arena of poetry. But I believe that the greatest work occurs when a certain self-forgetfulness is attained or least a fullness of self-possession denied to Sylvia Plath. . . . In "Lady Lazarus" . . . the cultural resonance of the original story is harnessed to a vehemently self-justifying purpose, so that the supra-personal dimensions of knowledge--to which myth typically gives access--are slighted in favor of the intense personal need of the poet.
If Birthday Letters is not a great book of poems because self-justification diminishes it, the same caveat must be applied to Plath's poetry.