Immersion said: "one thing for certain is that it brings "clarity" into the mix, since you hear the whole dynamic range more clearly..."
I wouldn't say this is certain. The perceived clarity is perhaps simply a result of a volume increase. If compression reduces the dynamic range then its not adding clarity.
I do quite a lot of mastering, and I'd say that a bit of well-placed EQ work brings clarity to a mix.
I'd have to agree with Anthony on both points. Would it add clarity if you asked an entire orchestra to play at a fixed volume for the duration of a piece? A lack of dynamic range can really bring up a listener's fatigue level very quickly. I was recently listening to a Natural Snow Buildings release that had almost no variation in dynamic range at all. My interest waned after about 10 minutes at the same volume level.
I find EQ to be far more critical to a mix than compression. Even better, why not introduce elements in an ambient piece that touch on all of the different frequency ranges? I find it hard to listen to some of my earlier pieces, which sound too midrangy to me.
I often use different reverb settings within a single piece, because it seems to create more of a "back to front" feeling than if I simply threw all of the sounds into the same reverb setting.