Actually, it depends on the target. Real life hostage rescue teams, SWAT teams, and certain branches of special forces often DO favor SMGs over assault rifles. An assault rifle normally fires high velocity round, and if not using certain types of ammunition, has been known to pass right through a "soft"(ie, non-body army wearing) target. It's also been known to pass through walls and risk hitting people across the street, in the next room, etc. In a "civilian" environment against non-body armor wearing opposition, a 9mm or .40 hollowpoint is more likely to stop the target and not risk civilian casualties that a high velocity 5.56 round. And of course, in some situations, a group might not be allowed to carry assault rifles(OR SMGs, not all private security agencies are authorized to carry more than a pistol), or might need something more discreet/lighter. I mean, the average cop on the beat doesn't walk around with an assault rifle or SMG, nor does the average FBI agent. Sure, in certain situations they will use a rifle or SMG, but if they just wind up in the middle of a situation, they may well only have their pistol. So really, the situation dictates the weapon more than anything, in some situations, trained personnel WILL choose an SMG over an assault rifle, and in some cases, they might only be ALLOWED to carry a pistol. I do get what you mean, generally speaking though-if a trained soldier/cop/whatever is knowingly going into a high risk situation, he's not gonna drop out of a helicopter onto the roof of a building carrying just a Beretta 92(seeemolition Man) or a pair of Desert Eagles(see:Resident Evil).
On a related note, dual wielding guns is pretty much never done in real life. Professional law enforcement or military personnel who are using their pistol will normally use a single pistol with a 2 hand grip barring certain extreme circumstances. Likewise, NO professional law enforcement agency or military unit carries Desert Eagles, despite what movies would have you believe. They are bulky, have excessive recoil, and have a low magazine capacity-real world cops or soldiers are more likely to have a 1911, SIG, Glock, or something similar.
---------- Post added 2013-03-29 at 12:30 AM ----------
In fantasy, sure. Realistically though, if the fate of, well, the entire world was at stake, someone who could call in a favor from an angel likely would. Especially when the alternative is to hope that a handful of guys(only half of whom are even trained in combat/war/survival/whatever) can cross the continent, secretly enter one of the most hostile and inhospitable places in the world, all the while bringing the ONE thing the person who lives in that place WANTS ever closer to him, while being pursued by his agents?
I dunno, it's like if we decided we wanted to nuke Iran by sending in 4 army rangers and 4 scientist guys who've never been out of the lab, start them in Taiwan, and have them carrying the nuke over ground...with Iranian intelligence aware of them travelling, rather than just, you know, sending a bomber to do it.