Sure, we could test long duration closed environmental systems in Earth orbit (the ISS doesn't come close). In fact, we would certainly do so for a time. Then what? Do we make the leap straight to Mars or could we use this technology to do some exploration, stepping further out with each iteration? Done properly, the technology would be useable for a variety of missions, not optimized for any one of them. Once you've proven a technology is good for 6 to 12 months, you've developed a reasonable confidence that it would also be good for a 12-24 month round trip to Mars.

Back in the early 1970s as Apollo was winding down, Werner von Braun was advocating going to Mars by 1982. I listened to Dr Ron Sega (Shuttle astronaut and engineering professor) as he described the obstacles of such a mission. He pointed out that we were no where near ready to go because of problems with life support and other systems. Rushing development to meet an artificial deadline often results in dead crews. The problem is that, since 1970, we really have not developed the systems necessary to go beyond Earth orbit. Further, we've lost much of the engineering cadre who made Apollo possible. There are a lot of lessons to learn and relearn. Going straight to Mars seems like an unduly hazardous approach, at least in my opinion.

As for the "been there, done that" attitude some people express about returning to the moon, I think there's a lot to learn from the moon still. I read in one book that the moon has a surface area about the same as Africa. In the 6 missions that landed on the moon, we managed to only explore a few square miles. Not only is there the scientific opportunities, there's also something to be said for testing systems and building skills needed for more demanding missions.