So did the Pro players of the 50's and 60's do nothing?
Yes the Pro slams can't be equated with the Open era slams. However, it is pretty much universally agreed that the Pro's were better players than the amateurs.
So if we care at all about tennis history - we want it to be accurate. Open era Grand Slam's didn't exist before 1968. So what do we say to that. Nothing that happened before 1968 was important in tennis? Or that the pre-1968 players weren't good players? Obviously neither of those. So how do we assess the best players in the world of the 1950's and 1960's? The only way we can is with the tournaments (and perhaps the head to head tours) that they played. The top tournaments that they played were the Pro. Slams - so we can discount those. And they have to be rated than the amateur slams at the same time. Almost everyone believes that Rosewall was number 1 for 1962 not Rod Laver. So if Pro. Slams rated over Amateur slams - then they were the highest rated events at the time. Hence, unless you feel that tennis should be totally discounted before 1968 - then you have to rate the Pro. Slams. There was no other standard to measure by at the time.
A very significant event was Laver's Open era slam of 1969. That showed that the best Pro was also the best Open era player.
Do we really think that Laver or Rosewall wouldn't have won most of the Grand Slam titles from 1963 to 1968 Australian - if tennis had gone open in 1961 (which it very nearly did)? If we do - then doesn't it just make sense to rate their top titles?