A comped or target/extended barrel semi-auto pistol like a M&P/FNS/Glock/XDm etc could be a decent duty sidearm. The size & weight might be a problem for 8/10/12 hour duty shifts but it's not impossible.
It's not whether it could or couldn't. It's whether someone would consider the merits regarding the pro's & con's and decide it was a better choice or compromise.
The overlong barrels and slides weren't the primary reason I wouldn't select a pistol model as a personally-owned duty weapon that wasn't originally offered as such by the gun maker. Granted, the days when we could get away from the 5"-6"+ duty revolvers made wearing them holstered a lot easier, as well as when sitting in cars and chairs. Shorter is better when you have to carry it for hours on end.
My hesitation would be in choosing something that deviated from what the gun company, and probably the agency, commonly considered to be service/duty weapons. This might attract some unwanted & unnecessary attention which is of arguable practical value when it comes to working guns that are going to have to withstand scrutiny by the public at some point.
Also, why risk having to get sidetracked during some court proceeding trying to explain why a target/comp pistol was selected, especially if it involved the use of a lighter trigger than the equivalent duty model pistol? Isn't the gun company's offered duty pistol sufficient for it intended role?
Id also not consider a 6/7 shot .357magnum or .44spl/.44magnum DA revolver ideal either. The calibers are powerful but, in 2014, a duty type semi-auto pistol is far more practical than a revolver. To carry 18 .38spl/.357magnum rounds vs 34-51 rounds of 9x19mm or .40S&W or .357sig is no real question.
Some "old salts" may say; "you only need 6 for sure" but these revolvers are not good for duty in 2014.
Yes, no & maybe.
The practicality of pistols that use magazines, versus DA revolvers that require the loading of cylinder charge holes, has been well accepted.
The capacity of the magazine-fed pistols is something that's still a subject often debated, though. The folks who like single stack .45's are just as fervent and strong in their perception of the advantages of low-cap .45's, because of the caliber, as the folks who look at sheer capacity as being the most critical consideration. Pick a "side" ... or stand squarely in the middle of the capacity "debate" ... and pick whatever suits your fancy.
Power? Really? That's so ... 80's.
It's a handgun, and it's chambered in one of the relatively low-powered service cartridges in common use nowadays.
You're right, though. It's 'no real question' when it comes to service weapons. It's still the mindset, skillset, training & experience of the equipment user that overshadows the minor details of the tools, themselves.
I wasn't advocating the return of the service revolver, and for a couple of reasons.
Firstly, it takes a lot more training & practice for a handgunner to be able to confidently, safely, accurately & effectively shoot a DA revolver. Pistols are generally easier equipment on which to train new shooters. Lighter triggers, better grip angles (compared to plow-share handled revolver frames and fat, sometimes awkward shaped grip stocks), generally better sights, lighter weight, less recoil (than Magnum calibers), etc.
No fumbling with bullet loops, dump pouches, speedloaders or speedstrips, either. No cylinder to swing out and hold in the fully open position while loading.
Secondly, being a pistol armorer is a lot easier and simpler than being a revolver armorer.
All of that said, though, I do miss the days when LE shooters learned their skills on DA revolvers. They generally made better all-around handgunners once they mastered the awkwardness of the grips and the heavier DA trigger pulls.
Since this topic is seemingly about what everyone may want to choose for themselves, though, it's not like anything other than personal preference, for whatever reason(s), is needed to make a selection.
At the end of the day, it's still just a service/duty handgun.
I'll offer one last bit of perspective, though.
If I had to choose a partner to make an entry through that looming dark & unknown doorway, and all we had were the duty guns hanging on our gun belts ... I'd always choose
experience of person over the type of
handgun he/she may be carrying. So, yes, I'd pick the experienced revolver carrier, who's BTDT, over the inexperienced person carrying 10 lbs of Velcro, 75 rounds of the latest gee-whiz ammo in a lunch box belt carrier, and the latest wonder pistol with threaded barrel, pop-up tactical displays and rader absorbing finish.
Equipment is just ... equipment.
Best, better, practical & sufficient ... are all words that require situational context and some further reference.