Very interesting discussion.
Ok, so no matter what grading is subjective as everything that is done by people. It is the people at the labs who grade; e.g., at HRD three clarity graders assess the stone and come up with clarity grade each independently from one another. If there is a disagreement a supervisor comes in and decides alone. While they use amazing technology (HRD really does), all their programs are only aids.
Just talking from experience diamond graders will often disagree; I have seen both agreement and complete disagreement. Just some examples although these are examples of color, about five different people graded my engagement ring's stone and here there was pretty much an agreement that the color was J, we then used a colorimeter (at least you can be sure in case of color and confirm with technology) which confirmed our results. Now when I sold my friend's engagement ring there was huge disagreement on color; in European terminology I graded it a Cape or Very Light Yellow, someone else graded it a Light Brown, a third person graded it Crystal, there is a vast difference between these color grades and all people involved were professionals. The stone definitely had very poor color.
Clarity is even harder as there is no program to assign a grade to a stone. There is a vast difference between an SI1 and another SI1, one I have seen had a humongous burn mark all over the table, to me this was terrible and extremely visible. Another may have two dark carbon inclusions on the crown facets which could be completely covered by prongs making the stone flawless to the eye when set.
Technically speaking it is possible for a stone to get a VS2 grade with an inclusion under the table; my stone is a VS2 with one inclusion under the table and one under a crown facet, neither is visible and one has to search really really hard with a loupe to even find the inclusion under the table. In the case of the stone concerned here that inclusion would make me instantly assign an SI1 grade to this stone.
Yes, an inclusion under the table would drop the clarity normally by at least one grade, sometimes many more depending on the inclusion>; e.g., one huge black carbon inclusion under the table would likely result in an I1-I2 if not I3. In the present case I have to wonder how much the program the vendor uses influences the visibility of the inclusion, this we cannot know but still I would refrain from buying this stone just to be on the safe side.
As to the question of color everyone in the trade has always advised me to get the best color you can get, so much is true, I personally own my J stone and love it but today I would select a higher color grade. That is not to say I would not keep my J stone, I would still proudly wear it and I often see amazing tinted stones at auctions, it all depends, I have no problem with it though I have to point out that an ideal cut stone with a very high color grade is truly amazing, icy white, which emphasizes brilliance in my opinion. I generally prefer higher than an H, G-H are very nice, F-s are probably a great value for how white they are, they are my faves.
That said it is not true that lower grades are not worth buying, they may be harder to resell and be tagged the so-called budget stones in the trade just like SI1-s but that may not be a bad thing at all. Same stands true for clarity; while I generally prefer a clarity grade higher than SI I have recently seen an amazing 1 carat pear with an F color and I1 clarity and I instantly fell in love with that stone and despite the clarity grade would be thrilled to own such a stone, now would I buy it to sell it? I would think about it but again many will sacrifice 1 C for the others. Such stones can have a market too.