Dollie,
I think materials certainly can vary depending on the context or use. For example a solid wood brick for travel would be silly. For that purpose foam will do. For a student who understands the nature of rooting and recoiling in asana a solid wood block provides a deep connection to that which is between human and earth. Things that are hollow; like words, promises, walls, do not have much umpf. Of course a hollow wood is lighter than a solid.
Props only for beginners? Not in my view. At home we have a nice assortment of thingies; mat remnants, chairs with the backs knocked out, belts of varying length, a pelvic swing for travel, 3minute eggs, solid wood blocks etc. Absolutely worth every penny we paid, IMHO.