I don't think infidelity can make a marriage better. There are much better ways to deal with marriage problems.
It's not the cheating that makes it better. It's the realizing that the spouses want to be together and working their bums off to make it happen, and happen healthily.
I haven't been cheated on, but when hubby and I were engaged we went through a nasty patch where I came home one day to find a half-empty apartment and a note saying "sorry". This was only a week after a new husband of a friend of mine did the same thing, without the note, and with quite a bit of theft of combined funds. The husband never allowed communication, he refused to let her fight for their marriage. My then-fiance did allow it, and the work we did on our relationship is what made us stronger than we ever were before. If we hadn't had "the troubles" as we call them, we wouldn't be here at all.
If a couple decides to do that work, put in the time, take it seriously, it can definitely make a marriage stronger than it was before.
But it (cheating) is not something to do in order to make a marriage stronger; that's laughable and I hope no one takes that post to mean that.
I personally can't help but think of the timing of Joanne Woodward and Paul Newman's marriage after his divorce from his first wife, and the fact that he had kids with the first wife...people want to put down pronouncements like once a cheater always a cheater, and women who have relationships with married men don't love the men...but there are ALWAYS going to be exceptions, and the above couple definitely seemed to be one of them. Of course, you don't know if you're part of an exception until the end...