[r00fus]

Did you use it with a headset or via device microphone?

The noise-correction on the older device microphones aren't as good as the newer ones (ie, anything that's Siri compatible has at least 2 microphones for noise-reduction).

So perhaps Apple does have a legitimate (although self-serving) reason not to deploy Siri on older tech - it may suck if you have lots of ambient noise or distortion.