Meshuggah not metal?
I had a hard time wrapping my mind around it at first too, but then i listened again and realized why it actually is the case. Admittedly, i do agree that Meshuggah and like bands are blurry borderline bands where any kind of distinction is all but worthless because, generally, they defy classification. Especially Meshuggah in this case, for giving them any genre label makes little sense because there either isn't one for them yet, or again, they just don't fit into any one category.
I never said a band is wrong for "not being Metal" or "Metal enough" or whatever. I just think it is helpful to be able to reference the different genres and sub-genres in terms of riff construction, drum construction, vocal style, lyrical/ideological bounds, and myriad other classifiers for the sake of specificity. Now, given that i am somewhat OCD to begin with, i suppose am prone to say that anyways.
As to your other points, i understand what you are saying, and yes, any artform affords a certain amount of subjectivity. But like ilyti says, the genre classifications are there, they deserve to have meaning. I personally like figuring out what something is based on what its origins are musically, what bands or sounds or genres influenced it, and how in the end these things effect the music enough to set it in a specific genre; and then, what that means for the genre. Because when talking about music i generally don't debate whether something is or isn't something, i want to talk about what it is based on where it came from, where its roots are, and what it meant/how it influenced others musically, ideologically, whathaveyou. Everything has origins in something, and i find that important, for the vitality of music is not just about the art itself, it is about the intentions of the art and that has very much to do with origins as well as the effects on other art around it.
Picasso and Georges Braque engineered Cubism - a new genre, if you will - but it had origins in something, that something being the meticulous study of form and geometry, and that origin therefore influenced the absolute intention of their works: to express and showcase a new study of form. It became a genre on its own through the ingenuity of the artists, but it was their artistic intention and interest in the origins of form that informed their masterpieces. Similarly, with music artistic intention and origins have much to do with what the outcome of the work is, but like Cubism, may be wholly disparate in actuality and (visually) audibly from any previous music form. Their work was influenced by one thing, but to the naked eye the previous origins may be blurred by the result. So, similarly, bands that love metal and are influenced highly by it, do not necessarily produce something of that genre; they produce something of their own artistic intention. If it doesn't fit within the boundaries of one type it is bound to fit in another, but it does not change their influence. However, the actual product may be very different from that which the artist originally intended, or it may stray from the genre parameters they hoped to achieve, but its a finished work.
I seriously doubt that Picasso and Braque associated working with cubes as an "ism" when they first started. It was probably more to the effect of "o, this is a cool idea." However, as time went on multiple people observed the properties and attributes of the art form and dubbed it accordingly; much like your Iommi example MDV. Then, everything falls in accordingly with a common origin, or it doesn't because its origins don't lie there. If someone started basing all form in paintings on and around the medium of circles exclusively, could it be said that they are a Cubist (we are talking very literally here)? Maybe, it certainly appears rather similar; but chances are they were influenced by Picasso or Braque and veered off in a different direction, creating something of a different genre. There are many variables here, and yes, some things really do defy classification. And yes, many don't. Doesn't make it good or bad, it just is.