Hi
Very often the only attribution that can be made with certainty is murano - this is the case with so much sommerso glass - however for the purposes of research its always good to know what maker it is (A.V.e.M, Seguso, Nason, Moretti) in that way we can build a good research database into the past designs of that glass maker - it can also be remembered that some people only collect certain makers within the murano catchment so they will always want to identify their pieces. I suppose it is also relevant that with a certain maker attribution the price tag also often rises but in the case of serious collectors that mainly counts for insurance valuations.
Adam