A manned Venus mission, in addition to giving a giant finger to the Russians, would certainly give us a great deal of knowledge regarding long-term space travel.
Also the loss of the Skylab program might have rammifications re: the ISS and the Space Shuttle. In the latter case, without plans for a space station (perhaps now regarded as pointless), the Military gets a purpose built reusable launch vehicle for payloads...
This might also lead to interesting rammifications re: the ISS. Without the experience of Skylab, perhaps the ISS is abandoned in favour of an international manned mission to Mars? The craft would be assembled in orbit and then launched in the early 21st century.
The thing about anything "International" is that it is extremely difficult to do. A Mars Mission that has a chance of reaching the red planet within 10 years of the first nickel spent on it is built in 2 pieces, with no orbital assembly. Unless the international partners settle for manufacturing the less visible components (electronics, life support, furniture, etc.), they won't be doing much.
For better or worse, ISS is the first real on orbit assembly experience anyone's ever had. Therefore, a Mars Mission launched without the ISS before it will either be simple, affordable, and launched from Earth, or experience the budget overruns ISS has had ITTL.
However, the Venus mission
could help provide experience for a 2 year Mars Mission. If they have the bright idea of spinning the habitation unit on the way, they have artificial gravity. That would be useful experience for the 6-month one way and 18-month surface stay of Mars.