I would say, it boils down to this ambiguity: Do we want to help players to recognize steel or do we want to help level designers to create beautiful levels?

1) As mentioned before, level designers can mess things up with both implementations of steel. Only with the current one, players can recognize it better.
2) Lix is a puzzle game, so it should be clear from the beginning, what is steel and what not.
3) We have to distinguish between pieces, that decorate other terrain/steel and decoration pieces that by chance sometimes reach over steel. Moss is of the first kind, because one cannot put is somewhere in the air and expect it to look good. The purple piece is of the second kind: One can easily create decoration in a completely empty area purely by putting such pieces there. So it is decoration, that is a priori independant of the steel, but may reach over some steel pieces.
With the decoration pieces of the first kind (e.g. moss) one might expect that it preserves the original terrain resp. steel. For the second kind (e.g. purple pieces), I would expect that it has one defined behavior, like any other piece that can appear without other context. So it should not matter, whether it is placed over steel or not.
4) I didn't know about that purple steel piece before. Would I have seen it in a Lix level for the first time, I would have expected it to be usual terrain, because I already knew it to be destructible and the rest of Lix works on the basis of wysiwyg.
Sorry, but I feel that adding steel and non-steel versions of one and the same piece is a mistake by graphic designers and only confuses players!
5) I haven't seen much steel decoration in recently made levels. Mostly it was used in the original L1 and in some early Lemmix levels. So I have to ask: Why do we want to decorate steel, when there is so much else one can decorate?
6) When I update old Lemmix levels that have moss over steel, I usually move the moss away from the edge of the steel. This way the decoration is preserved and players only discover the destructible moss by exploding a lemming there (which in many cases isn't terribly likely - Steel Works might be an exception here). And even when the moss gets blown away, the gameplay doesn't change - just one part of the levels looks ugly now.

PS: Something completely unrelated, but too minor for a separate topic: One of the L2-styles is currently called "medival". Shouldn't it be "medieval"?