That's not necessarily true. He's played his entire career in a park that, over the last three seasons, has depressed HR output by about 10%, more than likely playing a part in his low HR/FB numbers. Now, considering he'd be moving to Wrigley, a park that over the last three years has boosted HR output by about 10%, meaning that we could reasonably expect him to give up a few extra home runs, just based on the change in park alone.
What makes matters worse is the fact that Shields, for his career, has predominantly been a fly ball pitcher, with a career GB/FB of 1.15, only once giving up 25% more grounders than he did fly balls, and twice already has he come dangerously close to giving up as many FB's as he has GB's. A pitcher can get away with these kinds of things when he is pitching in Tropicana Field and has an OF consisting mainly of Carl Crawford and BJ Upton, but the outlook would be bleak in taking that same pitcher and moving him to Wrigley with an OF that has Alphonso Soriano on patrol, and others not named Carl Crawford and BJ Upton.
So in the end, we have a pitcher moving from a pitcher's park to a hitter's park, and that pitcher just happens to give up a lot of fly balls, and would conceivably be giving them up with a questionable at-best defensive OF behind him. His strikeout abilities would be a nice addition, but there's a whole lot else that isn't in his favor in coming to the Cubs.