Drafting is tailgating, and is dangerous under the best of circumstances. Unaniunced behind a stranger many times more so. I've personally had morons run into the back of me even when signaling stops. When I ride I will signal stops but I won't point out hazards unless I know someone is with me.
Tailgating is dangerous in cars, but, other than while stopped, have you ever seen a rider taken out by someone behind them? Seems to me the unannounced drafter imperils him/herself.
Since I've been over here, I've been impressed with the fluidity and ease with which pacelines comprising of complete strangers can form and dissolve out on the bike roads. People use bells, signal their intentions, and call out/ signal warnings about hazards. I'd like to think that this sort of pragmatic comraderie is possible everywhere among cyclists. What I've been reading here suggests otherwise.
I almost never have someone I don't know come up behind me (I know most everyone fast enough to do so)...
You probably hear this all the time, but: You Are Awesome!