Don't you think that Sonny uses Tessio more as an errand boy? He orders Tessio to recruit men after Vito is shot; he sends Tessio to see what was delivered at the gate; he orders Tessio to escort Mike to Louie's. Clemenza appears to be used by Sonny for more important matters such as training Michael and explaining to Michael the strategy for the Sollozzo meeting.
A lot of Clemenza's "importance" came from the fact that Richard Castellano was the biggest star (besides Brando) in the cast at that time. His role in the movie was beefed up because of that. The novel tended to show Tessio as being smarter. Tessio's role was somewhat lessened in the movie.
