Well, it was done in the past, wasn't it. Secondary burials. Votive offerings at springs that may have not been used for a long time. The list probably goes on.

The difference is, as you say, we don't need to leave any other traces of ourselves. We are everywhere already and very, very well recorded in most instances - at least in the 'western world'.

There would be absolutely no difference at all if a local school set up a website that said ... If we were to make a time capsule, this is what we'd put in it.

Children from a local school in Wales have also left their poetry (on stone) in a neolithic quarry on Mynydd Rhiw which has recently been excavated.. Why????

I suppose it's all about how you view and interact with sites. My personal opinion is that these monuments are not dead, their histories are still being written. I guess the local children may have been involved in the excavation and by leaving something of themselves they then become part of the quarries history. This inturn may foster a sense of ownership and belonging.

cheers
fitz

Lots more about the Stonehenge capsule here
http://www.e-timecapsule.com/stonehenge.html

Sounds like a real daft money making stunt to me, surely EH (and Unesco) will wake up and realise soon?