Basically, someone sends me an email requesting information on something and a lot of the time a standard answer will suffice. In quicktext you can setup several templates and just insert them an email from a pulldown menu (much like a signiture). However, the added benefit over a signiture is that you can use variables like [To=firstname] and [Text=HomeSignature] to automatically personalise the emails. I believe the pro (paid for) version also includes the ability to automatically add attachments based on variables in the quicktext template, which would be pretty handy for me.
I had thought of using signatures in Mail to setup template answers. I just wondered if there was anything available with added functionality. I haven't got into using apple script yet so that is not an immediately available solution for me.
Apart from this feature of Thunderbird I prefer using Mail.