That's a risky move... Paying for potential in a 6 year contract, this type of deal has worked in the past ( Derek Roy) but it also has blown up in a GMs face also ( Rostislav Olesz)
I think what's important here is Darcy and Lindy establishing Ennis' long-term role on the Sabres. In his final 30 games or so, Ennis didn't just play well - he dominated. Night after night, he was one of the best players on the ice. However, he wasn't facing top checking assignments on a consistent basis during that stretch.
Do they see Ennis as a guy who can go toe-to-toe with some of the better defenders in the league and be the cog of an offense? Or do they view him as a complimentary piece?