FabFilter

Hi!
I've just noticed some compressors including Pro-C have no sustain knob for compression. I think I've seen one that did have.

Is there a functional reason for this to be standard?

Opinions would be great. Thanks!

I've never seen a compressor with a sustain knob.

Here is the one that I saw:
www.waves.com/Content.aspx?id=171

I've only seen that one, I wanted why is it so rare and if one actually benefits from it.

Sorry, wrong compressor. I'm almost asleep so nevermind.