Huh? You wrote, "if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?" Do you not want a higher Q? Generally, people try for the
highest unloaded Q they can get, under some set of constraints.

Close spacing lowers the Q mainly because the current in the wire is no
longer radially symmetrical, if you look at a cross-section of the
(round) wire. That raises the RF resistance of the wire. For decent
(low-loss) form material, it's mainly the RF resistance of the wire
that determines the loss and therefore the Q. Generally, highest Q for
a given diameter and length is obtained by spacing the wire about two
wire diameters, center to center, at least for high frequency work. If
you want to use Litz wire, there's an optimum stranding...more, finer
strands are not necessarily better as you get to either lower or higher
frequencies. You should be able to find info on that, if you do some
searching.

There is such a thing as TOO HIGH a loaded Q. Let's say you start off
with a coil with unloaded Q of 500, and couple lightly to it with your
circuit (antenna and detector), so the loaded Q is 250. That means the
bandwidth at 3dB points, if you tune a station at 1MHz, is 4000Hz. If
you've tuned to the center of the station, your demodulated bandwidth
will be only 2kHz. Since the rolloff is gradual with a single-tuned
circuit, voice should be OK, but you'll be missing out on a lot of the
highs. (Mind you, it's not easy at all to get an unloaded Q of 500 at
1MHz!)

Using a very hack crystal radio--coil of about 3" diameter, antenna
just ten feet or so of wire, and an HP zero-bias schottky detector
diode--but into a low-noise audio amplifier--I've been able to listen
to standard broadcast stations a thousand miles away in the evening.
Biggest problem is getting rid of local stations...I'd use probably a
carefully designed three-resonator filter and a much better wire
antenna if I was serious about it.

Cheers,
Tom