I've never heard of people making freebase by heating ammonia and cocaine in a spoon, but it sounds a little sketchy. Heating ammonia (ie, ammonium hydroxide) produces ammonia gas, which is pretty nasty stuff. Granted, one can't fit that much ammonia on a spoon, so it won't generate much, but considering how often times one cooks many batches during a crack binge, the exposure could add up and might be harmful (especially since that exposure will be followed by significant additional stress to the lungs through the crack smoking process itself). The other concern would be residual ammonium hydroxide contamination of the crack if not properly washed.
Why not stick to the baking soda method?
