Tom didn't suggest that Vito give Carlo something important. He asked, "Should we give your new son-in-law something important in the family?" That's a big difference. Tom was doing his duty as consigliere: asking his boss if he wanted to bestow a favor on a new family member. He wasn't offering an opinion about Carlo's merits. He probably knew Carlo wasn't worth anything important, and was relieved that Vito said, "Never!"