My pulmonologist never told me to use a spacer, but my family doctor recommended it. The spacer allows you to get more of the medication, and you don't have to coordinate the breathing with the activation of the inhaler - which might be especially difficult if you aren't feeling well to begin with.

It's supposed to be especially important with a steroid (help prevent thrush). Now that I use Asmanex (dry powder inhaler), I can't use a spacer with that. But I still do with the rescuer. Makes it less portable - harder to slip in a pocket - but it seems more effective with a spacer.