Say, you have a pistol and an assault rifle in your loadout. Do you ever change weapons? In what situations you prefer to use pistol? What a pistol can offer that an assault rifle can't?
In ME2 pistols had better multiplier against armor while assault rifles were better against shields/barriers. I'm not sure if it's still the case in ME3.
Weapons in ME 3 don't receive any inherent bonuses against defense. To gain bonuses against armor, for example, would require an AP mod, AP ammo, or Drill ammo (EDIT: I don't think Drill Ammo exists in ME 3 SP, but it's been over a year, so I'm not sure...
)
(EDIT: The above paragraph is COMPLETELY WRONG. The Acolyte is a pistol with a massive multiplier against shields, for example. What I meant to say was that "pistols" or "SMGs" or "Assault Rifles" don't have inherent bonuses by type, but individual weapons sometimes do.)
As for when I would switch weapons, it largely depends on the character that I'm using and the enemies that I'm fighting. Most of the time, I carry a backup weapon that is almost never used. On the occasions that I need it, I'm glad that it's there, but the rest of the time I don't think about it.
One example is when playing a Shadow (this is an MP example, rather than an SP one) I use a Paladin and a Hurricane. The Paladin is the main weapon, using AP ammo, and the 2 mods are for power damage bonus and melee damage bonus.
Whenever I have to fight a bullet sponge that isn't safe to/can't be hit with Shadow Strike, I switch to the Hurricane, get into hard cover, and just concentrate fire on the target until it's dead. In the case of a Drell Assassin, I switch from a Black Widow main to an Acolyte backup for Phantoms or heavily shielded targets.
In SP, I never switched guns, because I used a Valiant most of the time. With the absurd "slowdown" effect of sniper scopes, there was no need to ever switch weapons. Even at point blank range, it was faster to kill an enemy with the sniper rifle than to switch to something shorter range.