Irrelevant. Many cases of Constitutional law start with thoroughly cynical motives (e.g. "How do I get a law against what I want to do struck down?" or "How do I get the damning evidence against my client thrown out?"). That doesn't affect the underlying merits of the issue.
As I noted earlier, the Consitutional issue is not with a specific number, but with the process of evading the "limited terms" clause through infinity-by-induction. The problem is that any challenge directed at any specific act of Congress is primarily focused on the former. The majority opinion addresses the infinity-by-induction argument by concluding that Eldred failed to present convincing evidence that this was in fact occuring (which implies that the Court may take a different view if Congress does indeed repeatedly extend copyrights to the point of preventing any post-Steamboat Willie works from maturing into the public domain).
Ginsberg, in a footnote:
Asserting that the last several decades have seen a proliferation of copyright legislation in departure from Congress' traditional pace of legislative amendment in this area, petitioners cite nine statutes passed between 1962 and 1974, each of which incrementally extended existing copyrights for brief periods. As respondent (Attorney General Ashcroft) points out, however, these statutes were all temporary placeholders subsumed into the systemic changes effected by the 1976 Act.
Judicial blinders: The old "It's not an act of Congress, it's just a 'temporary placeholder'" explanation.