You see, in the comic book series, Eugene was introduced along with Abraham and Rosita in The Walking Dead #53–and he was presented in a particular way: he was a man who had some inside information into the source of the outbreak, and who had managed to get Abraham and Rosita on his side by giving them hope of a cure.
As it turned out, down the line, he was full of crap. Eugene was a high school science teacher who had few marketable skills in the post-apocalyptic world and used a cover story about being a government agent in order to get aid and comfort from people more qualified for this lifestyle.
It’s hard to say just why they might blow his secret so soon, except perhaps that Rick Grimes, having already spoken with the CDC in Season One, will immediately not believe Eugene. They’ve already seeded a couple of clues about the Alexandria Safe Zone, so perhaps in spite of finding him out as a liar right away, they’ll find themselves headed in that same basic direction without necessarily having to be fooled into a fruitless trip to Washington, D.C.
Eugene has eventually made himself useful in the comics–most recently in a Walter White kind of way: he’s a guy who can make ammunition. That’s a useful skill as the apocalypse drags on–arguably more now on TV than in the comics, since the timeline is so much longer already on the live action series.
In AMC’s The Walking Dead, Eugene will be played by Josh McDermitt, best known to audiences for his roles on Last Comic Standing Season Four and TV Land’s Retired at 35.