Here is an article by Nippold from the University of Oregon questioning the connection.
PURPOSE:This article explains why it is reasonable to question the view that stuttering and language ability in children are linked, the so-called "stuttering-language connection."
METHOD:Studies that focused on syntactic, morphologic, and lexical development in children who stutter (CWS) are examined for evidence to support the following claims: 1) that CWS, as a
group, are more likely to have disordered or weak language skills ("language deficits") than children who do not stutter (CWNS); (2) that language deficits play a causal role in the onset of stuttering; and (3) that stuttering, over time, restricts children's language development.
RESULTS:Analysis of the evidence suggests that CWS, like CWNS, show the full range of language abilities (high, average, low); that language deficits are not associated with stuttering onset or persistence; and that stuttering has little or no impact on language development.
CONCLUSIONS:A connection between stuttering and language ability was not supported. An alternative perspective is that CWS have a compromised motor control system that makes it difficult to move forward in speech and that the tie to language lies not in a deficient linguistic system but in difficulty expressing the intended meaning via a fully functional speech system.