The reason behind this is that we've recently introduced a separate mail module supporting Amazon WS. If we're going to support more cloud platforms, we potentially will be having more email modules which would all consist of a template part. To keep that part maintainable we have splitted up the E-mail module with templates to two different modules. Besides that, in our opininion, the text template module offers more value than just for the e-mail module.
I think part of the reasoning behind this is that the SMTPEmail module will have platform level support, where templating will have community level support. These are my assumptions. I tried to check the actual support levels but all the page wants to tell me is "Could not create widget mxui.widget.DataView: TypeError: node is null" :).