Yes, a fighter that devotes his feats to defense has greater defense. But that is comparing apples to oranges. When comparing two fighters whose ultimate goal is to deal damage, the crossbow archer and polearm fighter are the two top contenders, and the archer wins out.
The archer's goal is certainly to deal damage, and has an edge compared to melee combatants. But if we're going to talk about balance between the two approaches then we have to look at other areas of combat beyond just DPR. Melee combatants usually have more durability or more AC or some other factor that helps offset the lesser damage they may have compared to a specialized archer build.
Not to mention that killing enemies faster actually saves the whole party far more HP overall and is a much more effective use of feats than on defense.
Sure if a party can pour out damage at a rate that drops most enemies quickly, that is an effective way to cut down on damage taken. I don't know if I'd agree that it always makes more sense to focus on damage output at the expense of all else, though.
How far spread out do your parties tend to be? Usually in the groups I'm in, players start within 10 feet of each other. And since the archer suffers no penalties for fighting in melee, he is more than in his right to do so, especially given that he can threaten OAs if he does so.
It varies quite a bit, depending on circumstances. But they do often start off together. They rarely remain side by side, though. Our ranged expert does indeed try to maintain distance and hit enemies from afar. And they react differently to events in combat.
The archer does not threaten opportunity attacks; he uses a bow and doesn't keep a weapon in his offhand.
Using my level 12 Great Weapon Fighter as an example, One thrown weapon for 1d6+5 or 3 ranged attacks for 1d8+1 (and having to drop his +1 greatsword). That is roughly 25% as effective as my normal round of melee attacks.
Can't he throw more than one weapon?
d4+5 vs 2d6+5 means the dagger is 62% as effective as the greatsword at an OA. If the archer chose to draw a rapier instead, his OA is 80% as effective as the great weapon fighter's. Compare that to the greatweapon fighter's ranged capability.
But what ranged weapon is your crossbow archer using? A hand crossbow? Or does he need to use his item interaction to draw his dagger one round, and then use it to stow the dagger next round? Does your DM allow you to load your weapon without a free hand?
Again you say the threat of them is what counts. The archer fighter can threaten them about 70% as well as the great weapon fighter can. That is still a very credible threat.
Not sure on the math, or the frequency of them, but that's fine.
Are you telling me that your crossbow expert just wades into melee? That he actually DOES threaten enemies with OAs? They didn't seem too big a deal to you at all, and attacking from a distance was something a lot of folks mentioned as an advantage...so how does it actually play out?
While an OA of 2d6+5 might be good against 10 HP enemies its not nearly as much of a threat to enemies with 100 HP. As such, provoking an OA from a front line warrior becomes much less a worry to most monsters as you reach higher and higher levels of play. This is mostly due to the fact that damage scales through additional attacks, but OAs only get 1. Hence the comparison.
HP fluctuate, no? At some point, every monster is a 10 HP creature.
The idea is about dealing damage outside of your turn. It's bonus damage. Any enemy that has to choose between moving and possibly taking an OA (or maybe two or more depending) or else taking his action in some other way is being forced to behave in response to the party. Maybe the bad guy has to spend his turn disengaging.
Again, this goes back to well designed encounters. If all your DM is worrried about is the total HP of damage your party can put out in relation to a group of enemies....then yes, combat will favor the high damage characters. Is that surprising?