I think that this type of art is great as any one can have a go; it can easily be a group activity and you don't need to have an artist's studio in which to create your work or a gallery in which to exhibit it.
Finally, whilst ephemeral interventions have largely captured the "land art" epithet, I feel that longer lasting interventions and indeed those installations operating at a larger scale or as part of landscape masterplans equally deseve our attention as artistic creations.