They pop up now and then. More common is that they get passed around between friends and shooting buddies. If you play regularly with a dozen guys, at least 1 if not 2-3 of the group will have a SAW (or 5

) stashed away.
Pros for Airsoft:
- you can lay down strings of suppressing fire without looking like a tool (i.e. someone with a 500rnd M4)
- no (or fewer) mags to carry...so you can go light with rigs/gear
- they look bad ass
- fit into mil-sim themes (i.e. a SAW per squad, support elements, etc...)
Cons
- heavy (don't buy a Star one that is plastic...you'll be seriously disappointed with the feel)
- sometimes fitting a battery can be a challenge (but very doable once you figure out what's what)
- box mags tend to rattle
- regardless of how "big" the AEG is...the FPS, range and accuracy will be about the same as other AEGs. I.e. an M4 can be made to shoot similar to an M14 similar to a G36, etc... Doesn't mean that all AEGs brought to the field are tip-top (they often aren't), just that lugging a 16lbs+ SAW around might not net any difference vs. a carbine depending on the game.
For speedball-like skrims...there's not much point since the sets are over very quick and it's usually a hose fest. Guys will regularly have hicaps...which turns every AEG into a SAW....and a 20 on 20 SAW fest is pretty boring.
For more tactical skirms...they're great if you're working with a small group. You can suppress/pin someone while you're buddies move to flank...you can hold off a position...etc... While everyone is frantically reloading their mags for the next set...you're kicking back 'cause you're ready to go.
For Mil-Sims...they're awesome. A lot of mil-sims are ammo limited based on what type of gun you have (i.e. Rifleman = 300 rnds....SAW = 1000 rounds....reloading opportunities might be a long ways away). That's where SAWs really come into play. Being able to support your buddies with some sustained fire while they're picking shots is often clutch. Ten riflemen = 3000rnds....8 riflemen + 2 SAWs = 4400rnds....and 50% more total ammo is a solid advantage....especially if the rules allow for teammates to share ammo on the field.
Having it is one thing...using it is another. How much you can carry while running/hustling is up to you.
Buy metal. The A&K 249's are made of "soft" metal. Carry handles, bipods will break (some guys take them off right away to save weight and to preserve some of the resale value). The box mag is ok...but kind of shitty if you're serious about things. I prefer the smaller CA "nutsack" mags. They're affordable though if you're only pulling it out now and then.
G&P and CA SAW/LMGs are solid. Inokatsu SAW/LMGs are outstanding.