I think the concept of nullification is somewhat more complex than the technical term. Nothing you have said is false, and I won't argue that the Supremacy clause is unnecessary. Our government is based on hierarchy, and it simply won't work for individual states to form alliances, tax other states, have wildly varying standards for a great many things, etc.
That does not mean however that might makes right, except for those who wield power. For everyone else "do as you are told" doesn't always sit well. In that case individual states do have some power which is not llegal nullification.
I'll give an example. If you look at I-93 through NH, you'll see a rather odd thing. The interstate stops, being a massive roadway and becomes narrowed, reduced to one lane each way at points. The reason for this is that the US wanted a "standard" for construction, which meant destroying the greatest natural area in the state. Now the Federal government hadn't the authority then to tell NH that it must obey and wreck it's natural treasures, but it did what it usually does and tried to compel NH by threatening to withhold funds, in essence blackmailing them. That's business as usual, but in the state of "live free or die", the NH motto, the citizens got a bit huffy about the whole affair and told Uncle Sam to take his six lane highway and shove it.
That caused some consternation because the Feds didn't want to back down, but they wanted a highway. So the compromise is what we have today, a functional roadway, and one of the most beautiful areas in New England preserved. It's a very satisfactory result.
Now what if the Federal government became so powerful, that it could have acted unopposed? You'd have smashed the area and got a shiny highway. I think that's what bothers many who support nullification. The relative power of one entity has increased, while that of the states and individuals diminished. What if the Federal government said that it had the right by way of the Commerce clause to build to it's specifications, and it's specifications alone? What if that highway was being built today? What would you say about the state refusing to back down? You might cite the Supremacy clause, or Commerce or whatever and claim that by rights, the states haven't any. The result would have been ruination on principle. In today's political environment that's exactly what I think would happen.
That brings us to nullification again, and a reason for increasing support. The best solutions aren't always given by the greatest power, and obedience to the law doesn't make right and people resent it.