So I've been arguing with my GM about this very topic (though before I saw this post as I just read it as of this posting), our group is comprised of mainly Ranged and Casters. Our GM has been running the Carrion Crown Adventure Path straight out of the book with almost no modification (extended the Time Frame to allow a trip to Carrion Hill after the first book). Our party setup is as follows:
Cleric of Iomedae who uses a Sword and Shield, but mainly has stayed back to Heal and Channel against Undead
Summoner with a Powerful Eidolon that has dealt major damage but hasn't been able to successfully take a Major Hit
Oracle Tengu using a Katana and a Daikatana that has normally stayed back and casted spells (horrible die rolls to attack = Many Whiffs)
And the GM made a Ranger (Ranged Specialist) to deal with the fact we were missing a "heavy hitter"
In the majority of our adventuring the GM has fudged many die rolls so none of us were significantly one-shot when facing major enemies (though the Eidolon was "killed" once toward the end of an encounter). The problem that I and my friend have not been able to convey to him is that in making the Ranger as a Ranged combatant hasn't quite filled the role of a person that can take the major hits that these major "bosses" have been dishing out. However as we have tried to explain we slip and say Tank, wherein he says he can't see using MMO Logic in a pencil & paper RPG, and brushes off the issue. However he is still not going to adjust the adventure to fit the group, and I just can't see a continuance of fudging die rolls so a character doesn't die as a viable option. He is also pissed when one of us talks about ditching a character to make a new one focused around being able to take hits.
In this respect I can see a definite need to define Combat Roles since there is going to inevitably be alot of Combat, but Challenge Roles are a different matter since they will change depending on the situation you get into (with a vast array of different situations within many different Adventure Paths/Modules/Homebrew).
I don't expect to get my own situation resolved anymore (in writing this reply) as the Campaign will either work itself out or fall apart altogether (if the GM gets too frustrated with the system not working in his favor or refuses to admit to MMO Logic as he puts it).