There are certain talents and equipment and things that specifically allow characters certain effects on triumphs, advantages, threats and despairs.

As a GM, do you only allow these effects when talents are in use?

I don't have books in front of me so a specific example is lacking here, but I'm sure there's a disarm ability somewhere. Would you allow a PC to say, spend a bunch of advantage to disarm an NPC if they didn't have that talent?

The reason I ask is I don't have the kind of mind that would remember that exists and would potentially allow a PC to do just that, which could devalue the talent, which I wouldn't want to do.

Anyone can do a disarm; it does not require a talent. The Djem So talent allows a disarm with a lower Advantage cost to activate.

I generally try not to make purchased talents a useless purchase by allowing others to complete the action without the talent but I also look at everything through the experience of 30+ years of gaming. If it's something that anyone can attempt, I will allow it but with setback dice where the person who has trained for it doesn't get setbacks and may possibly get boosts.

In the case of disarm, HappyDaze is correct. The book mentions a Triumph can be used to disarm an opponent so anyone can actually accomplish this.

It really just depends on the situation and the action being attempted, but I view those kind of talents as just another way to accomplish the same thing as a really good roll. I think what would really serve you best is just to read through the skill trees and get a sense of what talents are out there and adjust the amount of advantages/triumphs needed to accomplish something accordingly.The only skill trees you need to read are those of your players anyway, so it narrows down your focus pretty well.

Everybody else has pretty much said it, but I'll add my voice and experience to the chorus:

My goal is to give the characters the opportunity to feel as cool and powerful as the players want them to be. In general, that means rewarding quick thinking, clever problem-solving, and dedication to their character concepts with appropriate skill checks, Boost dice, etc. I also don't want to short-change players who spent a lot of XP to buy top-tier talents. These goals aren't mutually-exclusive, and being creative in your narration can help elide most issues of this happening. As long as they feel like their characters' skill and cleverness are represented, most players will be fine with whatever you come up with to explain their high-success, high-advantage roll.

Some players, however, have specific ideas for how they want their characters to succeed, and that's when you need to walk the thin line.

An example I actually experienced: the pilot in the group, when dodging TIE fighters in low orbit, wanted to make a roll to force the TIE fighters to collide with each other. However, this was within days of Stay on Target coming out, so I was already aware of the existence of the talent "Corellian Send Off" which covers this exact scenario. I didn't say no outright, but I did point out that, without the talent, any fancy flying on her part would be the result of her roll. She made the check and succeeded, with enough advantage that I ruled she could force one of the TIEs into a collision with an unmanned satellite.

In this example, I had a couple of things going for me. First, the player instantly understood my reasoning for why I couldn't just allow her to attempt it—not just because of the talent, but because the Narrative Dice System doesn't really support that kind of exacting specificity in its rolls. It's more effective to say, "I want to try some fancy flying and see what I can pull off," then adjudicate the results, than to say, "I want to do this really particular thing," and have no wiggle room when you fail.

Second, her roll was good enough for me to feel comfortable giving her something similar to what she wanted. In this case, she had something like five advantages, which would be enough for an unarmed crit in a fistfight. So I figured it would be enough to pull off a critical hit without using a weapon (a.k.a., the ship counted as "unarmed") and the best way to narrate that was by forcing the TIE to fly into an obstacle. Likewise, she was cool with that reasoning.

Now, if she'd rolled two Triumphs on the check, then I'd have felt compelled to allow her to manage a Corellian Send Off-style upset. But that's a pretty high cost, when someone with the talent could have done so with a simple success on what's probably an easier check. (I'm AFB, but I think it's a Hard check, as opposed to the Speed+Silhouette+terrain-based difficulty of a normal piloting check.) And though "Corellian Send Off" is an action, other talents that could fall under this situation might be maneuvers, while making a talentless (har) skill check is almost always an action.

But probably the most important takeaway is: I explained my reasoning for my decision, and we worked out a compromise. If I had just said, "No," the end result probably would have been much different.

Knockdown talent comes to mind on this one.

I'll allow a Talent effect to occur without the talent at a thematically appropriate moment from time to time.

The big difference I lay down is who gets to decide.

For a RAW example, look at something like Spending a D-point vs. Utility Belt. Both the base rule and Talent allow the player to produce a previously unrecorded item. The difference is Spending a D-point requires the GM to agree, where Utility Belt applies conditions, but in exchange doesn't require the GM to agree (well, not anymore than the GM has to agree with anything that happens in game at all).

So by that reasoning I'm ok with say, if you have a Lightsaber and an attacker rolls a despair, I might allow something like Improved Reflect to occur, even if you don't have the talent. I'll probably demand a D-point or something, but the thing is it's my call as the GM to allow that or not, and if the player wants it to happen, and I'm not sure it should, he'd better present a good reason for it to occur.

However, same situation, but now you as the Play do have Improved reflect. An opponent generates sufficient threat, and you say it happens. So it does. Now if there's some reason for it not to the tables are turned and it would be on the GM to provide a darn good reasons for it.

And that's the difference. When there's no talent, the GM is just being a good guy and saying "Eh... I'll allow it this time since I think it will improve the game and story if that happen now, go for it" and can just as easily and more often say "Mmm... I think you'd need the talent to do that at this moment." When you have the talent it's a hard "Yes, the rules say you can do it, so do it." and when you can't it's gotta be something like "This requires a working lightsaber, and yours in in six different pieces, one of which is in the rancor's stomach... so no, you can't do that, but try something else."