These people can only do it a few times. If they do it too much their multi-rank privileges get revoked and all roles act equivalent to their main role. The more they do it, the closer it moves to that point.
Basically, being able to play and learn other roles should be considered a privilege, and one that can and will be taken away if you abuse it.
My worst case was I got someone insisting on a trade for his main position, after the game he admitted he traded AWAY from his main position because we had a Teemo support and he didn't want to lose placements. I will never trade again.
All things considered, I say it's too early to say to trash it or keep it either way. So for now, 5 ranks makes sense.