Defense is important but vastly overrated for RF. RF make their money with their bats, specifically their power bats (or should). Jeff Francouer has one of the best RF arms in the game, makes exciting plays at the plate and plays pretty good defense but no one thinks he's elite because his bat sucks. Heyward is nowhere near the depths of Frenchy level but he's not a $150-200M player. He cut way down on his strike outs and lost his power. Could the power come back? Maybe. Potentially.
Think about it this way: take a RF who is.270-.280, gonna hit 15hr, strikes out 100 times a year, hits 30 2B, steals you 15-20 bases and has an OPS in the mid .700 and plays average defense. What is that guy worth? Would you be willing to go 7 years $84M for him? I wouldn't but let's assume the answer is yes. Then take Heyward, same stats but with excellent defense. Is he worth 7 years $154M? To me, the answer is definitely no. But, let's assume the answer is yes. Then you are paying $10M extra per year for excellent RF defense as opposed to average RF defense.
Does that make sense?