Let me add a little to this to be more clear... and maybe sound like less of a cromagnon angler
if the tippet X rating relates to fly size, then doesn't the whole arrangement fall apart when large, heavy, weighted, densely tied, epoxied flies or sinking lines are used? I've often heard people talking about how they had to got to a 2x to cast that fly, or it was so windy that... etc. So the X to fly size relationship is really loose once fly design is considered.
Also the stiffness of various monofilaments, flourocarbons, varies quite a bit. We all know that stiffness plays a role in casting and turnover.
The breaking strength has already been talked about but it can't be completely separated from this discussion as thick tippets are frequently used to prevent breakage.
If it's purpose is to aid the angler in deciding which fly or tippet to use, then I see a good 'rule of thumb' value there.
But I don't see it as being necessarily better than a good gut feel for using a thinner or thicker tippet for experienced anglers.
The X rating also applies to whole leaders, e.g. a 3x leader. I believe that leader is based on tippet size graduating back to the butt. I tend to match the butt to the fly line first then taper forward to the target tippet size, which can be achieved by lengthing (adding sections) or shortening (removing sections).
Butt to line matching does wonders for energy transfer.
Just a few more thoughts on the topic...