So if no aliases are defined and I send an email to <myname+sometext@gmail.com>, it will simply strip and discard "+sometext" (which is what I have seen), whereas if I define an alias as "myname+other", it will recognise an incoming email to <myname+other@gmail.com and process it as a valid address.
Both cases are received by the same user account at gmail, whether an alias was predefined or not. The alias case simply allows the user to filter incoming messages based on the alias.
So I can indeed cite this when I talk to BT as a feature which the synchronoss/BT bug will break.