Since Rainbow Six Vegas 2 offers you two slots for a full-sized assault weapon in addition to two equipment slots and a handgun of choice, your trusty sidearm doesn't get a whole lot of attention throughout the game. However: these pistols contain unlimited ammunition, an unrealistic but attractive feature that should entice more strategic players to practice with their handguns. Not to mention that the handgun is your only option while dangling from a rappelling rope...
Each handgun can be fixed with a silencer in-game, which can be removed at any time as well. The only exception to this is the Taurus Raging Bull, a .44 Magnum revolver that's often mistaken for the more common .357 Magnum. Rapid fire is not an issue, as your hands are steadier than Ted Nugent's after a midnight peace pipe. Since the cover system is very intuitive and very necessary to complete the heavier densities of 'Realistic' terrorist cells, the key to staying alive is smart cover and knowing your weapons.
Personally, I'm always up for a pistols-only round, a tradition that hails back to SOCOM: U.S. Navy SEALs for me (and Counterstrike for many others); a good choice of handgun and decent aim help make for a successful Terrorist Hunt, no matter the difficulty nor ratio of bad guys to good guys. If the heat is too much, make sure you have a 12-gauge shotgun strapped to your back...just in case. Otherwise, use your handgun to take out any windows -- which are apt to shatter and distract you in the middle of a serious firefight -- and border sentries, before they notice you peeking from behind a distant packing crate. If you can manage, try to work through at least half of the map with just your sidearm; not only will it impress your squadmates, but you'll be soon be a master of rifle recoil and close-quarters encounters.
If the Terrorist Hunt co-op is not what you fancy (blasphemy!!) and you're a versus mode XP whore, then the pistol can still come in handy, so spend some time choosing before deploying. When you're pinned against a corner, you can blind fire (remember: infinite ammo!) to keep your opponent's head out of sight, giving yourself time to catch a better glimpse and perhaps a better firing position. In the High Stakes Mode -- a free update that was released one year ago this week -- this is particularly useful, as your view while in cover is limited to more realistically portray your character's perspective.
While it's true that firearms are not for everyone and present a significant amount of danger to an unprepared user, games on the other hand are for everyone and pose no dangers whatsoever. Just the same: be choosy when you're asked to holster a sidearm. Don't snag the most powerful or the first one on the list just for the sake of jumping into the action. Chances are that if you play enough, your handgun will see the light of day at some point and possibly get you through that last, elusive 'Realistic' four-player Terrorist Hunt.
One day, developers will wise up and include the Sig P226 in their combat games. Until then, there's no universally correct answer to the ultimate question in every combatant's mind: which gun should I strap to my hip?