I think that there is a lot of confusion here as to what 'stonehenge' actually refers too. What we are talking about is a whole landscape which has no borders...it reaches out into the surrounding areas for miles around and cant be understood in isolation. We now understand that it extends welo beyond the WHS. Thus just because the tunnel entrances are not next to the henge itself or within the WHS doesnt mean that they wont do damage.
As to whether its adding to our knowledge, i think we have found more new monuments or features in the stonehenge envelope in the last few years than in all the time since serious study started. In just two weeks last summer we found five new monuments and sites that were not known before, at least one of which is likely to be of extreme importance and could rewrite our understanding of the area.
The Stonehenge area is nationally important because there were extrordinary things happening there from the mesolithic right through to the bronze age. Much of our understanding of these periods has come from the area due to the amount of activity that occured there and the often excellent presivation.
Contrary to what you might have been led to believe, you cant just do remote sensing on an area and then claim you know all there is to know about it. Remote sensing will give you a vague outline and depth of a feature but it wont tell you what it was, when it was made, what was done there, etc, it'll just give you something you can compare to other plots and allow yo to suggest that this featuremay besimmilar to that one.
Regarding not allowing people near the stones, thats sad but given the number of peoplewho go and the damage they were doing its understandable. Back in the 1970s there was a bloke there who would hire you a hammer so that you could chip bits off to take home!!!!
Anyhow, the current situation is untenable....not just for the site but due to the massive traffic tailbacks there. Some thing needs yo be done to reduce the traffic levels. A tunnel is a stop gap solution....when traffic increases again you cant just add another lane to a tunnel!
As it happens im on a jolly on thursday, taking a load of undergrads to stonehenge for their landscape archaeology module......making the most of it while its still there