The Thalmor are supposingly gaining influence in Skyrim but from what i see they fail completely on their missions. Their failed missions include:

The plot against the college of Winterhold (Ancano is killed at the end)

The try to learn what is going on with dragons (they are ridiculed by the dragonborn and Esbern)

The distruction of the Thalmor guarrison at Northwatch Keep(during the mission missing in action)

The failure of Captain Valmir ( A Thalmor agent) to cheat you during the quest "siege on the dragon cult"

Their failures continue in Solstheim were they fail to find Stalhrim despite all their efforts

All i can say is that if they want to replace imperials, they must create better secret services

Was Captain Valmir actually a Thalmor agent? He's High Elven, but I don't remember him being Thalmor?

And the common element that caused all those losses is? The dragonborn. They've proven that they are better than the Empire. They drove the Blades into near extinction and defeated an Empire that should have been much stronger than them. I think they are good enough.

In every game everyone is "useless" except for the player.

Yeah, he has some notes on him which I think mention the Thalmor being after the mask or something in those lines.

Almost every. In FO:NV there were some important ones. What makes an NPC or faction important/useful? : Consequenses on actions. And the NPC/faction makes game-changing desicions during the game.

I'm wondering how you can spell Destruction wrong, as it is a skill in the game...

And they arent useless! You can level up your skills on them and their corspes!

Edited that for you

Agreed. The Thalmor have proven more than capable of dealing with The Empire and continue to show superiority in ability over the Blades, Stormcloaks, and even able to manipulate the Khajiit and Bosmer into their alliance. The only one they can't beat is the legendary player character.