 Frontier Airlines parent Frontier Group Holdings Inc. ULCC, 6.57 issued a letter to Spirit Airlines Inc. SAVE, 2.90 shareholders, urging them to support the air carriers agreed upon merger deal. In the letter, Frontier Chairman William Franke and Chief Executive Barry Biffle say the recently amended Frontier-Spirit deal offers Spirit shareholders value well in excess of JetBlue Airways Corp.s JBLU, 5.77 illusory proposal, which lacks any realistic likelihood of obtaining regulatory approval. 