There's no great wisdom in suggesting that playing in slams is different from playing in other events, or that in general, the more matches played against each other, the better for comparative purposes.
A truly valuable contribution would be demonstrating exactly how head-to-head records might be logically employed for some purpose other than determining which one of the only two tennis players on a desert island is the better of the two. That is, how can head-to-head records be used to rank any group of three or more players? How can head-to-head records be used to rank players who have never played against each other?
For my part, I don't see how head-to-head records can logically and reliably rank even a tiny subset of the field, such as Nadal, Federer, and Djokovic. Nadal has a huge edge on Federer, so he must be ranked well above Fed, correct? Federer in turn has a slight edge on Djokovic, so that means the Djoker is in third place, I suppose. But wait: Djokovic has a pretty respectable head-to-head against Nadal, clearly better than Fed's, so he must leap ahead of Federer, yes? Now throw Sampras into the mix. He has no head-to-head against any of these players, except a blip of an 0-1 record against Federer. How does he fit into the head-to-head-determined matrix?
A > B / B > C / C > A head-to-head combinations will always exist. Now add something that can't be even be directly compared, such as Σ, and the problem is compounded.