Is a latex additive necessary?

Yes and No

Latex additive is a liquid that you would add to your thinset mortar instead of water when mixing. It makes the thinset more flexible once it dries and makes it very sticky. So why would you need this stuff?

Glass tiles are nonporous. This is what makes them great for use outdoors. You don't have to worry about water absorption and then freezing issues. However since they are nonporous it is a little bit harder for the thinset to grab and hold these tiles. The latex creates an amazing bond between the glass and the thinset that will make you feel safe that your tiles are going to stay on.

Obviously if you are using any other adhesive the latex additive is not necessary. Or if you are using ceramic or porcelain there is really no need for it. But it doesn't hurt to use it since it does add the flexibility of the thinset.