Towed decoys are fairly common in modern fair forces, as they should be more effective than chaff owing to the decoy being able to mirror the host plane’s movements for prolonged periods of time.
The downside is that they do impose some limits on the host aircraft’s own speed and manoeuvres when deployed. So it’s a toss up between being able to push your airframe to the max to try to evade missiles, or to limit yourself to the operating boundaries of the towed decoy. Also, even if you use the towed decoy, you typically cannot deploy more than one at a time, so you are only giving you yourself a 50-50 chance, and probably not even that, as it would be pretty easy to build on behaviour laws to help missiles distinguish between plane and towed decoy. Hell, I would expect that to be standard on modern missiles.
Normally towed decoys are more useful for multirole aircraft on strike missions, where the flight envelope of the fighters would be significantly limited by ordinance carried anyways.
Fighters on air superiority missions would generally prefer to rely on their own agility more.
Although I think towed decoys are pretty much yesterday’s tech, and are being supplanted by modern EW systems able to achieve better results through signal processing and agile broadcasting while not imposing any limits on the host plane’s flight envelope.
