Beetee is a District 3 victor who won the Hunger Games long ago. He's a genius with technical gadgets and contraptions. Katniss is blown away by his intelligence:
There's nothing patronizing in [Beetee's] voice, and yet I can't help thinking he reminds me of a schoolteacher about to ease children into a lesson. Perhaps it's the age difference, or simply that Beetee is probably about a million times smarter than the rest of us. (25.18)
But you need more than smarts in the arena. You also need physical strength, and Beetee doesn't have that. Since his last trip to the Hunger Games, he's gotten sharper, but he's also grown older and weaker. He's injured early on in the Quarter Quell, when he attempts to get his hands on a special weapon, and again later when he tries to use it. He's not a fighter anymore and can't really defend himself against the enemy the way Finnick, Johanna, and Katniss can. They need each other if they hope to survive.
Things get more complicated when we learn that the reason Beetee knows so much about the weapon he risks his life for is that he's the one who created it. He's part of the rebels' plan to bust out of the arena. Katniss doesn't know this when she considers him as a potential ally; she must just have good people instincts.