To be a deep threat one has to actually catch the football consistently, not just run really fast downfield. While Darrius Heyward-Bey does possess exceptional straight line speed, does he have the football awareness to understand the nuances of the Patriots' playbook and not just be a one-trick pony/useless decoy or the hands to consistently "go up and get it"?
If not, then, I'd rather the Patriots just stick with Donte' Stallworth, to be the team's straight line speedster. Even at his advanced age at least we know he can actually catch a deep pass and knows the playbook (a little).
So far Heyward-Bey hasn't consistently shown anything other being a colossal Al Davis bust. Perhaps a culture change is what a player of his talents needs. Though its likely he really just can't put it together regardless, I mean he did get cut by the Raiders of all teams.