Sorry, how many programs which you class as "Trojans" add what you
define as a "backdoor", given that a "backdoor" is generally
pre-compiled code which allows access via previously un-announced or
commonly unused connection methods? Malware doesn't typically ADD
backdoors, it comes shipped with them, thus the classification
Trojan.Backdoor, as opposed to just Trojan. Many of the more common
Trojans these days are Worms, Trojans, and Backdoors and some are Viri
too. The reason is simple - short of breaking the kernel process
scheduler it is useful to be a Trojan when present as an active virus.
Similarly due to the current nature of desktop and server side
application logic, most viri are unsuccessful without being worms -
although this may change in a few decades as applications become more
data driven and automatic. Nothing will ever substitute a full
description of a particular malware's actions in describing what it
does, unless you expect malware authors to start conforming to
standards.