While your point may be valid, the counterweight is that the WTF round robin is among the 8 most highly ranked players in the world, who are most often legitimately the best. The slams can be a mishmash of very lowly ranked opposition and one or two from 1-20 range. Kinda evens out
I don't quite see it that way, because if the "seedings" in grandslams were to go as to plan and the top 8 players made it to the quarter finals, then you would have to beat a fellow top 8 player 3 times over a best of 5 sets. That's win 9 sets over them. Ask Djokovic whether he prefers facing Nadal at the o2 arena indoors over the best of 3 sets, as opposed to Nadal outdoors in potentially hotsunshine in Australia over the best of 5. Go and ask him what he feels is the easier proposition.
There are also a lot of unvalued lower ranked players that don't get enough notice, particularly on MTF. MTF quite frankly does not give a shit about anybody outside the top 10, period, let's be honest and I am qualified to say that because I've seen it for with my very own eyes when they rule out the lower ranked guys and then end up looking stupid but instead make excuses and try to start suggesting the higher ranked bloke is out of form, choked, played wack at crucial moments.
Raonic, Janowcicz, Youzhny, Verdasco, Isner, Kohlschrieber, Monfils and even mental giants like Dodig. All of these player's have the arsenal of game to beat the Berdych's Tsonga's and Ferrer's of this world. Even Del Potro if he's having slightly off game. They can also provide as much if not more resistance to the top 5 games, you know those player's that bring their very best to slams? Play inspired tennis? Jerzy Janowicz is a classic example of this. Rises to the occasion and atmoshperhe/vibe. The only reason they aren't ranked as high as the Berdych's Tsonga's and Ferrer's are because they aren't consistent over the duration of a season.
What my point finally comes down to is, the top 8 player's in the world on paper are not necessarily ALWAYS the top 8 best player's in the world, they may sometimes just be the top 8 best performers throughout the duration of a tennis calendar season. Is Gasquet for example warranted of being 1 of the top best 8 blokes in tennis? No. Ofcourse not. There's somany guy's who'd have given Del Potro, Djokovic and Federer better games.