Right now you end up with something that looks like this if you attempt to make a symbol class transparent that may include overlapping or coincident features, by setting transparency on individual symbol class elements:

Yes, you can set transparency on the entire layer itself (effects toolbar on appearance tab), but this doesn't work if you are using anything other than single symbol symbology and wish to retain opacity in your other symbol classes. A control on a per-symbol-class basis would be helpful.