is there any reason (safety?) why someone should rename the ASP.NET Session Cookie Name or is it just a senseless option of ASP.NET?
I think its mainly a matter of taste. Some people/companies want control every aspect of their web apps and might just use another name for consistency with other cookie names. For example, if you use very short one-character parameter names throughout your app you might not like session cookie names like ASPSESSID.
Security reasons might apply but security through obscurity is rather weak in my opinion.