Well Shirou refers to it as a spell when he hears Archer use it:
What you said of it being a means to activate his "tracing" that's what spells do. They say multiple times that spells are only used to connect oneself with "the world" or whatever.
I am aware that shorter spells are a sign of superior magi, they explain that with Caster, saying that her ability to use magic without saying any spells is considered remarkable.
I was also joking when I called Shirou as "master magus", I just chalk up him having such a short spell and being so unskilled as another one of Nasu's many breakings of his own rules. Though with "Trace, on." he was probably just thinking "it sounds cool."
I could be wrong, but I have no intention of arguing it here. I want this thread to stay useful rather than turning into a mess of people bickering.