If that defender can't switch between melee and ranged, with credible powers for both, that shows how codified his role really is. That fighter tends to be stuck at the front, melee weapon in hand, with weaker options for ranged attacks. In other games, with less codified roles, that fighter could have chosen to be a real killer with a bow.
As I said, I'm not really the best person to answer this, my 4e experience is fairly limited.
But, in 3e, to be a "real killer with a bow" meant that my fighter wasn't much of a front line fighter. If I'm burning bow related feats, at least until I get very high level, I don't have the extra feats to make me also great at melee combat.
Competent, yes. Sure. But great? Nope.
Earlier editions didn't really suffer from this, mostly because there simply weren't any build options to take. Your guy would likely be a great bowman because he had a high dex/low strength and a magic bow. If he was the reverse, high Str/low Dex, then he had a magic sword and a bow was something that got used in a pinch.
See, my experience is very different. Characters have always tended to focus IME. If I was building a brute force character in AD&D, I pumped up Str, then Con, and then whatever was left went into Dex. Sure, I had a bow, but, that was an after thought, not a focus.
Then again, in AD&D, because monsters tended to be so much weaker relative to the PC's, it was easy to be a great melee and great ranged character. A bow +1 meant that you could pump out 10-20% of any baddies hit points in a round when giants only have about 45 hit points.
But, there's more to it than simply combat role specification. The inflation of the monsters, and the massive scaling back of the PC's relative to challenges has had a much larger impact IMO. Characters specialized because they had the tools to do so, and doing so was rewarded by the system.
Codification of combat roles simply recognizes that fact, not emphasizes.
After all, what defines a class? If you look at classes in any edition, what do you see? Hit Dice, weapons allowed, armor allowed. Combat abilities out the wazoo with a dash of out of combat stuff for a few specific classes. The character's Combat Role was defined out of the box all the way back at the beginning, the fighting man fights, the wizard is artillery and the cleric heals/provides support.
The only real difference is that 4e split the "fight" role into "striker" and "defender" and then based character abilities on that. Since characters are hardly limited to a single role as most abilities allow for a great deal of flexibility within that role and between roles (fighters are defender/strikers or controllers depending on what you take forex) codifying roles hasn't really changed anything.
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Mark CMG - I gotta go, but I haven't forgotten your question, I just wanted to answer this first.