The roles are really a bit of a pity on the game design level though, some of them establish minigames where most of the party cannot participate meaningfully in, due to crippling overspecialization, they can only make things worse.
Lets see, personally what roles I'd define...these roles are non-exclusive, you can be multiple.
Out of combat:
The Radar - The one with great senses, exotic senses and generally dealing with the Knowing of whats going on. Darkvision, high perception skills, Scent, Sense Motive, Search, Detect Magic, Detect Evil, Tracking are low level capabilities of this role. Arcane Sight, Blindsight, etc are staples at high. Note that only persistent detection capabilities are counted, you cannot be party radar effectively if you have more limited usage, on the basis that you can't turn stuff on until you know theres something to react to.
Benefits from being redundant, and can have multiple characters participate in different ways(packing different exotic senses, etc). A necessary role, since awareness is necessary to prepare for and deal with hazards and opportunities.
The Sneak - The one with concealment, subtlety and doing things without being known to be doing them. High stealth skills, Darkstalker, Invisibility, Hide in Plain Sight, Lead Sheets, Bluff, Sleight of Hand, Silent Spell. You don't need to do this all day(though its nice to be able to), only to do something without being known to.
Benefits strongly from complete coverage, but also optional, since you can just take things straight on the table.
The Negotiator - The 'offensive' social abilities, used to make people do things you want, but they don't. Diplomacy, Intimidate, Charm Person, Dominate, etc, as well as language ability and telepathy. Also only necessary in bursts, so limited use abilities are fine, success rate is critical for coercion based negotiators, as without Sneak options, the opposite outcome can be obtained.
Nice to have some redundancy(so the whole party doesn't sit out while one person talks), but ultimately you only really need one.
The Transport - Getting from Point A to Point B. At low levels, you have mobility skills like Climb, Jump, Swim, Ride, and Survival(to get to the RIGHT Point B), at higher flight and teleportation dominates. Requirements vary, but until one movement can deal with everything between A and B(e.g. teleport) you're going to need them persistently available.
Extremely useful to have redundant, unless one member can transport another(as with teleport), one person getting stuck would have the whole party stopped.
Obstacle Removal - Getting rid of, or ignoring obstacles preventing particular actions. Some overlap with Transporter, this consists of Open Lock, Disable Device, Dispel Magic, some of the immunity/resistance effects, Freedom of Movement, etc. Usable in bursts, but frequency required can vary widely.
Redundancy is optional, but where obstacles are common, extra removal ability can be valuable, as does a variety of removal options that covers different obstacles.
The Observer - Getting information on specifics. This covers a combination Sneak/Radar character scouting, scouting minions, Scrying, and a variety of divinations. Useful mostly in bursts, persistent observation ability can rip any semblance of mystery from the plot to bits, or for the matter the plot itself.
Redundancy unnecessary, one way of getting remote information is as good as another.
The Library - Getting general information, mainly associated with Knowledge skills, decipher scrip, spellcraft and divination spells, but also gather info. Mainly acts as plot keys and backs up Radar and Observer roles by allowing action based on the information.
Redundancy is useless, but there is a wide spread of knowledge types and multiple people can have them to ease the load on any one's skill spread.
I think that should cover most of the out of combat roles.
