[Ultraviolet]

The point of Talents is to allow a character to be highly skilled in a narrow field of skills rather than investing broadly in Attributes and by association being skilled in many fields.
Such generalists are IM HO trite and boring, and makes characters in a party too similar.

Imagine a campaign about spacefarers. The engineer, diplomat, and planetary surveyor all rely omn IQ based skills, so they invest heavily in IQ and may end up with similar stats. So they all have the same defaults in all fields not covered by their skills, also in those related to the other guys' fields (unless we talk about defaults from skills).
So when the engineer needs to machine a part to fix the reactor, and uses pure IQ default to Machinist he might as well let the diplomat get his hands dirty because they are equally skilled.
However had they used Talents the Engineer's might have included this skill.

I know this is putting things on the edge, since skills within same fields often default to other skills. But still.

Talents are a thing I (once I saw them in 4th ed) sorely missed back in 4th ed. There were a few advantages back then in 3rd ed that could help you: Mathematical Ability for techs and engineers, Musical Ability, Double Jointed for thieves and stage magicians.

And I use them a lot.
Say in my Cliffhangers campaign, the action-oriented explorer type can avoid being a total brain as well but cover his most vital science skills with a talent, save some points and focus on being tough, have good reflexes etc.