I've thought a lot about the subject of the gravestones and think it really isn't a cut and dry thing. There are several different factors that mean different things where different survivors and victims are concerned. For instance, in the case of Fleet, he died destitute with no family and his grave lay unmarked for many years. Such an important figure in Titanic lore shouldn't have been left with an unmarked grave. While I also think its a little cheesy to add the name of the Titanic organization that placed the marker, I'd rather see SOMETHING marking Fleet's grave than nothing at all.
Quite different is the case of Milton Long. The family had placed the markers they wanted and as Milton was an only child, there were no family members left to object when changes/additions were made to the family stones.
There are cases like Mary Kelly (Mrs. J.J. Heslin) and even Nella Goldenberg, where the graves are unmarked yet there are many family members that could afford to place a headstone. If the families, for whatever reason, have decided not to do so, I can't imagine an organization doing so in good conscience. That decision should rest entirely with the family. If approached by an organization about placing one, the family should have complete say-so in what is inscribed on the stone.
There are others--Mrs. Wren in Canada, Nora Fleming in Pennsylvania, etc. that left no family and have no markers of any sort and weren't major figures in the Titanic saga. I'd think it a noble deed to place a marker at those graves, but again would hope they would simply give the vital information on the individual with perhaps an inscription saying "Titanic Survivor." Anything beyond that is showy and grandstanding.
Just my $.02