One of the arguments not necessarily put forth, is the fact that these stone circles and megaliths which have been around for thousands of years and managed to survive earlier destruction, have an independent 'right' to be protected given our present understanding of past history.
Their conservation relies on guardianship either from the appropiate body or individuals, if, occasionally we could move the argument from the 'ego' of the person arguing ;) to an understanding of how we should protect them in the best condition possible for their 'own sake' and future generations, than modern belief systems should not appropiate such sites as their own. So therefore not arguing against different present belief systems, or the fact that they use such sites as part of the ritual, only that there should be no damage on the site....