Have not seen it before (was there last in March 08). If anyone has a picture of it, or is going there soon, please post it on this thread.
I don't mind discreetly placed plaque's in the Berg. There is of course the one in memory of Roland Smith (who ran a resort/lodge in the Champagne Valley) in Roland's Cave. I also accompanied someone to the summit of Cleft Peak once to look for an old plaque of their grandfather (he was simply someone who loved the Berg), which was found after a bit of searching.
The problem with them is that there shouldn't be man-made artefacts in proclaimed wilderness areas (by law and definition). As long there are only a few, placed away from trails, it's manageable. I don't think the one in question should be right at the Marble Baths. It should be tucked away somewhere where you have to look for it (like a geocache of sorts). It can infringe on people's sense of being out in the wilderness and it can also encourage placing of more plaques, which if it becomes more popular, will become an issue and will lead to them being removed.