This is the reason I'd choose an SMG over an Assault Rifle.
http://tvtropes.org/pmwiki/pmwiki.php/Main/BriefcaseBlasterTo boot, you can fit one in a large smuggling cyber part, making it much, much harder to discover. I believe the conceal ability becomes a -8 for the limb it's stashed in, and without a proper cyberscanner, you'd never detect it. And the Cyberscanners are just Millimeter Wave machines, which IRL don't penetrate skin. You'd never know it was there without dismantling the limb.
If you're on a run anyway, grab a Praetor, load it full of APDS and mod it with Gas Vent III, then put a gyromount on your strength 9 (or 11) cyberarm for yo bad self. Now you've got upwards of 11 points of recoil compensation, and you look like any old joe when you walk into the Corp headquarters. Great for a stealth mission that might require some firepower once your shmooze your way to the back, or for the Face/Assassin that might make a loud getaway. And wouldn't it be great to talk your way into the security server room and just hose it down? I think that would be a fantastic way to start a smash-and-grab.
Also nice for a truly terrifying high class bodyguard.
And don't forget, most corp security relies on SMGs. The bullets don't go through good walls like an AR might, meaning less damage that the corp needs to clean up. That means that if you need a combat reload, you can probably find ammo on the guys you've gotta drop. Sometimes that's important.
As for Machine Pistols, with a Lined Coat and a concealable holster, you've got a concealment of -4, good enough to pack full-auto fire nearly anywhere you go. Don't forget that Suppressed opponents have a -Hits penalty to all their actions! This is great for making a getaway or for buying time for your fellow runners to act. Also pretty awesome when you're on the back of a motorcycle, the suppression fire hits everyone in the target vehicle! Machine Pistols also fit in Cyber holsters, making them quick-draw champs.
Shadowrun it not like our old friend DnD. The biggest and loudest guns have their place, and that place is the Barrens and the jungle. It is not downtown Seattle, where Knight Errant is gonna wandering-encounter your machine gun lugging butt.
