Why did the rebellion allow Katniss to enter the games a second time in The Hunger Games: Catching Fire if she was so important? It would seem like a much safer approach to have just taken Katniss to a secret location as soon as anyone knew they were planning to have the Victors re-enter the games. Instead they allowed her risk accident or death, only to be saved by a helicopter flying through a miraculous breach in the dome. Is there a reason for this approach that I missed?
I think there are a few reasons behind this.
Firstly, it allowed Katniss Everdeen, the symbol of the revolution, to be seen and admired by all the Districts. Her appearance as a Mockingjay before the 75th Games began would have been inspirational to all the fighters in the Districts. It sent the message even though she couldn't avoid the Games and couldn't avoid another fight, she was taking it head on and being herself - not the girl-to-be-married Snow wanted her to be.
Secondly, it allowed all the undercover games entrants to be collected from the same location. Although they knew each other before the Games began, this game the Rebellion the opportunity to get all of its former victors in one place at one time and evacuate them all.
Thirdly, and perhaps most importantly, this all happened on live television. If Katniss had joined the rebellion, or rather been rescued by them, in the Capitol, before the Games began, the Capitol would have suppressed the information and released a cover story. With Katniss disappearing in front of the world and the Games - which are used by the Capitol to display their superiority over the Districts - being blown apart (literally), the world knew how serious the Rebellion was and how strong it was. The message this act sent was powerful enough to risk Katniss.
This is a theme that gets explored further in the third book, when they bring Katniss out into the warzone on different occassions. Although they don't really want to risk her life, they want her close enough to the action for her to be inspirational and symbolic as the tough freedom fighter the rebellion's members think she is.
To conclude, imagine famous scenes from real-world history, such as the Tiananmen Square image - and now imagine the powerful effect they've had. I believe Katniss' appearance in the Games was intended to have the same effect. Whilst the first two reasons above are important, the third reason was the crucial one - her escape from the Games well and truly demonstrated the strength of the Rebellion for the first time to both the Rebellion's members and the citizens of the Capitol.
As per my speculation, the rebels allowed Katniss to enter Second time to send message to all 12 district about rebellion. Do you remember her dress conversion into Mockingjay, that must be a message to all about the coming rebellion. In the Hunger Game world, districts can't contact each other but the games are a good way to send messages to all districts.
My second thought is that they didn't get the right opportunity before. Katniss is now a star and people see her, follow her and a TV channel also covers her. Her disappearance need to be pre-planned. The 75th game was surprise for all, so they didn't have time for her escape plan.
But I more strongly believe the first one where the rebellion wants her to be a Mockingjay symbol for all by participating.
After the victory tour, President Snow sees the effects of Katniss on the Districts and the riots. He was planning to eliminate her, but Plutarch Heavensbee offers to take her into the games to give the districts a message: Even the strongest among all are weak against the Capitol and even they can be killed easily by the Capitol
So the Quarter Quell game had changed, and all previous winners are invited.
A speech between Katniss and Plutarch in the 74.th Hunger Games Victory Ball:
So, the only chance Plutarch had was to help Katniss and to support the riots to put her into the games. Otherwise, Snow will get rid of her in some other way.
But in the books, the situation is quite different. At the Victory Ball, after dancing with Katniss, Plutarch Heavensbee shows her his watch. For a second, Katniss saw a mockingjay on the watch, which disappears afterwards. Plutarch tells Katniss that the watch is important and valuable.
After her rescue from the Arena, Plutarch tells her the reason he showed her his watch was he was expecting to see Katniss as a Mentor of new tributes, not in the arena as a tribute.
So, Quarter Quell tribute plan all belongs to President Snow, and the rebellion has nothing to stop Snow. Only thing they can do is arrange other competitors so they can help Katniss stay alive in the arena. Johanna concusses Katniss to remove the tracker chip so she can be rescued by the rebellion and not captured by the Capitol etc.
There is a simple reason for this and one of the reasons they don't let Katniss know about the plan. It's that if she lives, she can be a symbol to hold up to all in the Districts. If she dies, she becomes a martyr, which is an even larger symbol they can rally the Districts behind. Either way, they have what they need. Katniss is just a pawn in the whole ordeal. Albeit a willing pawn, but a pawn nonetheless.
Not to mention we would have missed a couple hundred pages and a couple of hours of awesome if that were the case. The third book would essentially just merge in to the second...
But, in all seriousness, I like Andrew's answer. I also don't think Katniss would have let that happen. She's the kind of character that tackles things head-on, and will do whatever it might take to avenge the past and make way for the future. Whether she is to be a martyr or a hero, she is the character who will be either, but she would not be okay with being stolen away before she could get her message out to the world.
It's not just a matter of the rebellion "taking her". It's also herself as a character.