As a Great Lakes wreck diver, I am with Mike Ferrara. I have enough "stuff" sitting around. I don't need to add to the collection. I do like my digital photos that take up nothing but memory on my computer, LOL!
I read stories about port holes coming up and that type of thing in salt water. I have little problem with that, but I feel that is a bit extreme to go down with a hacksaw and take the RDF antenna off of a WW2 U-Boat? That is a part of the dive that someone will not get to enjoy now, but that is tempered with the knowledge that sooner or later the Atlantic Ocean would have claimed it anyways. So, if in salt water where artifact recovery is legal... knock yourselves out!
In cold freshwater, the wrecks are far more interesting as time capsules. I routinely dive 100+ year old wrecks that look much as they did when they when down. I am appreciative of the laws protecting said wrecks for that reason. These wrecks will be there 20 yrs. from now looking much the same as they do now for our enjoyment. I would rather take a camera and minimize diver impact on the wreck. One of these days, I will want to go inside further than I do now and it is nice to know that I will be able to see much of what other people have seen about life in the late 1800's through 1900's.