If you're asking if it's better to have a pistol and two mags or 9mm/.45ACP, or two extra mags of 5.56mm, consider the following:
Is your rifle likely to jam, break a firing pin, or become unserviceable during an engagement?
Are you more likely to run out of ammunition for your rifle?
In either of these situations do you think that you will be able to effectively use a pistol or another weapon?
Honestly, you'd have to look at the statistics for this. But you can't change the weapons loadout based on the mission or location - even though in urban combat pistols are infinately more useful than open field - because having the same thing on your person at all times reduced thinking and reaction times when the pressure is on.
I can look up and hit a stationary target with a 9mm at 20m at the range, but I doubt I could repeat it under fire (mainly because I haven't seen combat yet, and don't have good practice).
So the question is, do you issue people with pistols (which are light, compact and have moderate usefulness) or issue them with an SMG (which is bulky, heavy and expensive) which could perhaps be useful as a room broom or close engagement decisive weapon?
No.
The pistol will remain until someone invents a newer small arms weapon that is accurate, weighs very little, and is more accurate and useful than a pistol. Which, in reality, would pretty much be a miniature assault rifle!