Let me put it another way (you guys aren't understanding the essence of my question).
Why not have a room that is FULLY DAMPED?
The short answer is, reflections done right are your friends.
Reverberation is desirable for the sake of timbral richness and a sense of spaciousness. If that were not the case, concert halls and recital halls would be anechoic or nearly so.
As further evidence that at least some reverberation is desirable, if sufficient reverberation is not naturally in the recording it will usually be added by the recording engineer, so that the recording has not only timbral richness but also conveys ambience and a sense of the recording venue's space, even if that "space" is synthetic.
In the playback room, there is a "competition" between the acoustic space on the recording and the "small room signature" inherent to the room.
One of the reasons we don't want to "fully damp" the playback room is that then the ONLY direction we'd be getting the recording's reverberation from would be directly from the speakers. And it just so happens that the WORST possible direction for reflections to arrive from is the SAME direction as the first-arrival sound.
Ideally we want the reverberation which is on the recording to be delivered from all around, and if we're talking about two-channel, then the in-room reflections are the CARRIERS of the reverberation which is on the recording. So what we need to do is, MINIMIZE the playback room's "small room signature" cues WITHOUT killing off the recording's desirable venue cues.
Here is the secret: The "small room signature" cues are most strongly conveyed by the earliest reflections, whereas recording venue cues are most strongly conveyed by the reverberation tails. So what we want to do is MINIMIZE the early reflections without STARVING the beneficial later ones. When done correctly, clarity is not degraded yet we are immersed in the soundscape on the recording.
As a reality check, let's step back and take a look at a good seat in a good concert hall. In the front third of the hall, and in the middle section (as opposed to near the sides), is generally where the good seats are. In these seats the direct sound is still loud enough to be clearly dominant (something that is usually not an issue in home audio), then there is a considerable time gap in between the direct sound and the strong onset of reflections, and those reflections take a while to die out. THAT is the basic scenario we want to recreate in the playback room, though obviously on a smaller scale because our reflection paths are shorter.
So imo it is not as simple as looking at RT60 times, because those tell us nothing about the early reflections, which are the ones most likely to be detrimental. In particular, we want to minimize lateral reflections arriving withing the first 10 milliseconds, which is feasible in most rooms, without killing off the later-arriving reflections. This is a problem which has multiple possible solutions, which I won't go into here.
There is one other thing which comes into play, and that is the spectral balance of the reflections. It is desirable for the reflections to have approximately the same spectral balance as the first-arrival sound. To a certain extent this is a function of loudspeaker radiation patterns, but room acoustic treatments can play a huge (and not always beneficial) role. In general room acoustic treatments remove more energy at short wavelengths than at long ones, so they tend to not only make the reflections quieter, but also duller, as far as spectral balance. This can do more harm than good!
You see, the ear/brain system classifies reflections as such based on their spectral balance matching up well with an earlier initial sound. When there is a significant spectral discrepancy between the direct sound and the reflections, the ear/brain system literally has to work harder to classify the reflections, and over time this "increased CPU usage" results in listening fatigue and even a headache.
And once a reflection is no longer recognizable as such by the ear/brain system, it ceases to be "signal" and becomes "noise". So the EFFECTIVE noise floor may actually be HIGHER if the room has too much damping, because sounds which would normally have been "signal" are now "noise". And what we are losing includes those desirable reverberation tails which convey the venue's ambience cues.
Of course there are ways to address the loudspeaker/room interface which do not have these downsides. But my guess is that when people have removed their acoustic treatments and heard an improvement, some of that may be due to restoring the high frequency energy which should have been present in the reflections all along.
Sorry for the long post; the topic is far more involved than what I've written here.
But to sum up: Reflections done right are your friends.
