They're great glasses to use when trying out new techniques, too. First, they aren't as horrifically expensive as some of the other glasses and they tend to work really well together. They stay clean and don't blur and react the way some other colours do.
Also, I know that Effetre Black is actually a very dark purple but I like the way you can see that hint of purple when it's layered very thinly. It gives the black and white beads a little colour which I find a bit more appealing than plain black and white.