cost vs science
Normally I'm all for spending money on science, I say we should spend as much as we can. But for the billions this would cost, it doesn't seem worth it, especially in the press.
Everybody loves the twin rovers, they've lasted years, and years past their expected date. This on the other hand would have a similar price tag (research, prototypes, testing, more testing, etc is the majority of the cost), but only get an hour and a half of data. While I realize that's a ton for scientists, and can take years to go through, it's a lot of risk for relatively little gain.
And there is a lot more risk to a flying machine than a rover, after the rover lands (crashes), as long as it boots it can do science, one of the rovers has been running for years with a bum wheel. A plane on the other hand would have to initialize almost perfectly, if the wings don't lock, or don't bend far enough, or the engine clogs, or the sensors jam, etc, then you have a very expensive unprotected dead weight falling to the ground.
Wouldn't an inflatable be more cost effective? It would need only a small power source to move, and give more controllabillity, you could hover over an interest spot on the ground, or in an especially fluffy cloud. And as long as it inflates and holds pressure, it won't crash, even if every part on it fails except a camera and the balloon, it would still move in the atmosphere and it would be useful.