surveyor47, first off it's nice to meet you
Secondly, I understand your point on the capacity of the Glock. I've carried over the years the 17, 19, 26, 30, 32, 36 and even the 21. They are great firearms and they have my full recommendation. On duty I've got the 21 .45 ACP.
I'm going to come at my decision to use a revolver from a different angle to hopefully explain my rationale. When I first started, we used the S&W 64 .38 special. We had the 6 rounds of .38 special +P in the gun of course...and that was it! We trained with speed loaders of course, but didn't actually have them available on-duty. So you had your 6 and that was it. In fact, we didn't get them until about seven years later. So, with only six...we really, REALLY got good with them. Many agencies were similar 'back in the day'.
As an instructor we often had access to many statistics and such. For example, the FBI did a ten year national study between revolvers and semi-autos in relation to hit %. What they found was that when officers were primarily armed with 6 shot revolvers they were hitting their target (bad guy) 78% of the time on the first shot. That is a great % considering they are under duress which usually involves an adrenaline dump. This can cause tunnel vision, auditory exclusion, loss of dexterity in the extremities (fingers), spike in blood pressure, pulse and respiration etc.
When semi-autos came into vougue, the first hit % dropped to 48% which is where the term 'spray and pray' came from. This does not mean hi-capacity semi-autos are a bad thing. Just that for a short time, good shooting practices went to the wayside.
I carry off-duty for the same reason as anyone else, personal/family self-protection. An off-duty officer isn't likely to be wearing body armor or have his radio or less-than-lethal tools with him i.e. O.C. spray, Taser etc. Therefore descretion is often the better part of valor i.e. don't run around with a big 'S' on your chest thinking your super cop. Step in when necessary of course, but then many private citizens would as well in similar circumstances.
I don't put myself in bad situations if I can help it. I don't hang out in bars, strip clubs or dark alleys. If something goes south on me, it will likely be the same sort of 'something' that could happen to Joe-private-citizen. And statistically, the number of shots fired in most shootings is very low I don't feel outgunned with what I'm carrying these days. Having many rounds is nice of course, but being able to accurately place those rounds is of more concern to me. I'm not a world shooting ace, but without patting myself on the back I'm a step up from the average officer since I've had a LOT of practice as an instructor (which is the way it should be if you're going to teach).
Just my view. Hope some of it made sense.