An experienced cutter and tailor on this board is of course welcome to correct me, but my understanding is that rock of eye places more emphasis on the fitting process relative to the other methods. The freehand method of drafting a formula is less "controlled," if you will, so it's necessary for the cutter to see how the first draft of the garment fits on the client, and then edit from there. With the other methods, the tailor is more likely to arrive at something reasonably controlled, and the image of what he's trying to strike in his head is more likely to overlap with what the fitter might have in his head. I don't believe that's so with rock of eye.
Both Rock of eye and block pattern manipulation obviously comes before any fittings (on the first suit) and the value of the fittings has nothing to do with how the pattern was drafted but with how well it fits and how hard someone wants to try.
Rock of eye just means that you remember what the basic ratios are for what goes where. You supply the block pattern from your memory.
People seem to recoil at the idea that something starts as a block pattern because they tend to forget that people come in more or less standard configurations - same number of arms in the same place, etc.
As ET pointed out, drawing directly onto the cloth is a sales technique and as Ed mentioned, the proof is always in the pudding. There are enough stories that start with block patterns and end happily.