The Plot to Steal Soviet Planetary Data
astronomy.com
astronomy.com
The data itself might not be valuable, but being first to have it could be valuable for propaganda purposes. Perhaps the Soviets wanted to avoid a repeat of what happened with Luna 9.
Luna 9 was the first probe to survive landing on the moon and send back images. Their plan was to release these images to the world press with much fanfare, emphasizing how it was their superior science and engineering that allowed for this.
Unfortunately for them, Luna 9 landed and started transmitting at a time when the moon was not visible to the Soviet monitoring stations. It was visible from Jodrell Bank in England, though, and the English were listening. The public affairs officer at Jodrell Bank had earlier worked in the newspaper paper industry, and noted that the transmission sounded like the early FAX machines that the papers used. The observatory called the Daily Express to ask for help, and the Daily Express sent their FAX machine over. The Luna 9 transmission was put through it, and it worked! They got images from the moon.
The Daily Express got the story out, with the images. The Soviets were not pleased that the first look they got at their own probe's moon images was in an English newspaper, and getting scooped meant that the West got to set the tone of the conversation, so it focused on the the science, not on the superiority of Soviet science and engineering.
Maybe I missed it?
However, this site spells it all out: "Radio systems of Soviet Mars and Venus probes" http://www.svengrahn.pp.se/radioind/MVradio/MVradio.htm
The whole article is based on this document, which about as informative as a series of photographs taken as Roswell ... with the lens cap on.