12" 45 RPM records: What's the story?

The main advantage is better signal to noise ratio and secondarily, better frequency response (because the groove wiggles are stretched over a longer length of groove.)

The downside is that there is much less playing time per side (which is the answer to your second question.)
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The main advantage is better signal to noise ratio and secondarily, better frequency response (because the groove wiggles are stretched over a longer length of groove.)

I'll also add "better dynamic range"...those 12" 45 RPM records were simply awesome compared to regular LP sound. I used to buy tons of them (about half my old collection). The most noticeable improvement was in the bass. Less feedback issues especially at higher SPL levels. I am not sure if many night clubs still use them today, but these used to be the pro's choice to blast out quality hits over high end club systems and draw crowds. (A quality of sound production that few could match at home)

One advantage is more exercise for audiophiles than 33 1/3RPM....You will notice in general audiophiles with Turntables are in a little better shape than those with cd only and those that listen to tuners more are a little plumper than the average audiophile...JD

As Shadorne says the ones I used to own always claimed better dynamic range and it seemed that way to me. Although I always felt like I was being taken advantage of because of the shorter playing time.