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Alas, there are many sources of noise other than read noise.
Janesick (2001) notes the following on just thermal noise
depletion dark current
diffusion dark current
substrate dark current
surface dark current
backside dark current
dark shot noise
dark current non-uniformity
dark current spikes
Despite zero read noise, an L3CCD sensor can't differentiate a thermally liberated electron from one generated by a photon, hence close attention to dark current minimization is essential.
There are other on chip noise sources
spurious charge
residual surface image
residual bulk image
clocking luminescence
pixel luminescence
diode luminescence
cosmic rays & radiation
excess charge
blem spillover
seam noise
cosmetic defects
There are also off chip sources, eg light leak, pre-amp noise, ADC quantizing, clock jitter & EMF.
While read noise may be minimal with L3CCD there are other technologies that are equally promising eg Clipper chips that also have sub electron read noise.
There is no doubt noise reduction is a good thing, but with very faint objects, the signal just isn't arriving quick enough for the above noise sources to be non trivial.....it would seem better practice to me to gather decidedly more flux (ie long exposure) than constantly run the gamut of noise sources with multiple short subs.
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