This is what I think. Each team has a certain potential, some obviously higher than others. It is up to the skill of the player that makes use of the team to its highest potential. SOME teams are really easy to play out to 100% of its potential because the team itself doesn't have much potential, while others require a lot more skill to execute to its full potential because there are many more options, and it is up to the scenario of the battle and the skill of the player to make what the best option is.
Maybe this isn't the best example since I'm not amazing at battling, but if we compare, say, an offensive Rain Dance Team to a more balanced team, it's probably easier to reach the team's highest potential with the rain team because most of the time things can pretty much go one way (set up rain and try to sweep), whereas with a balanced team, there may be many more things to consider (like what to sacrifice, what to switch in, when to do certain things) and would thus require more skill.
So, when looking at it this way, we see why people stealing good teams from RMT don't usually get the same results as the original posters who are high rated, since they do not have the skill to benefit from the full potential of the good team.
EDIT: Oh, maybe this is an better (more extreme) example. A team of Ubers vs a team of NFEs. The team of Ubers obviously (well, generally speaking) has a much higher potential than a team of NFEs, and so, even if it's a horrible player who can only reach maybe 10% of the team's potential, he might still be able to beat the person with an NFE team even if the NFE team player is doing 90% of his team's potential. So, this means, between the two, TEAM is ultimately more important.