Wow.  The OP has the temerity to question the big mags...and is immediately attacked as a "Fudd", a Lliberal and all other manner of monster.  And now Gatehouse posts an in-depth journalistic point-by-point analysis of why all the points that he claims are used by the enemies of freedom when they talk about big mags...are wrong!  "Don't tell us you're right, 'cuz you're not!  You're wrong!  We're right, and if you disagree, well, you're just an old senile commie  sympathizer, so...so...so there!
I don't know about the west coast brush; unless it has hands and fingers and a mind of its own, I doubt that it is all that much more "grabby" than the northern Ontario stuff I grew up hunting in.  I expected that when moving through it carrying a rifle, that there would be some degree of "hanging up" going on.  I fully agree that the sling was the number 1 offender, and so of  course I removed it when hunting the stuff.  When hunting, I want the gun in my hands, plain and simple.  A sling or strap was useful for carrying the gun from point A to point B; I'm sure somebody will stridently point out that the sling can be a shooting aid, and it sure can...but using it that way in the kind of brush you're whining about is just silly.  Shots in that stuff are quick and close, not something you need to wrap into a sling to make the shot.
There have been times when I had hunted the wide-open spaces of the West, or of Africa, or the tundra/taiga  of the  North, and I have loved the chance to just leave the sling on the rifle.  It was nice...but the simple fact is that sometimes that works comfortably, and other times it doesn't.  So, obviously, sometimes when it's  useful you leave it on, and sometimes when it's a PITA you don't....just like a big mag.  
Even among the people who feel that way about slings, there's disagreement about 'em.  A big, wide leather sling with the gunmaker's name emblazoned on it, along with some basketweave and some lacework and a couple cartridge loops and maybe a little knife  sheath, or a military-style sling that if laid out as a single  straight strap is about 15 feet long...well, those things are way heavier than they need to be...just like big mags.
To me, a big mag isn't going to increase or decrease the "snagginess" of my rifle significantly...but it will stick out right where I want to wrap my hand around the gun...and it will interfere with taking advantage of the sort of natural  rest I like to use, like treelimbs, boulders, etc.  I'm not guessing...I know it will; I have committed the sin of getting old, and have  spent plenty of that wasted time hunting, including some of it spent hunting with rifles that have big mags sticking out.  I find them a PITA in many situations, so...being old and dumb...I no longer  use them in those situations.  They have their advantages, but most of the time, those advantages  are far outweighed by their drawbacks. 
It was stated above that they'd be  useful for shooting groundhogs...?  Really?  A great day of groundhog shooting might see you take a dozen shots, maybe 20, spaced out over a number of hours.  It would involve a fair bit of walking and carrying, and likely all shots would be very deliberate and taken from a rest.  Sounds to me like a perfect scenario for a single-shot rifle, and the exact  opposite of the situation where I might find a big mag useful.  But to each his own...and that, boys and girls, is true even when the other guy believes the opposite of what you believe.
We have guys who spend big bucks on a lightweight rifle, and then spend more big bucks on replacement parts that lighten it further.  Titanium this, magnesium that, carbon fibre here, skeletonization there...cool, I just doubled the cost I have into this rifle but I've shaved  off three  ounces!!!  Sure, many of  them then slap on 3-pound riflescopes and 10-round mags weighing a  pound and a half, but it's their choice to do so.  I get that, but...what is so horrible about asking for their rationale?  
If not for this thread, I'd never have  thought about shooting at herds of pigs charging across my field of fire...or about some types of rifle competitions...but I don't do that stuff, so why would I?   Those things don't factor into my decisions regarding rifles, but having them brought to my attention I can certainly see that big mags would be useful for them.  They're obviously not just for cold-weather porch plinking, even if that's all that I find them useful for.
Seriously, a lot of you guys sound literally desperate to not merely explain the big mags...but to make it sound as if they have no drawbacks.  Everything has advantages and disadvantages, and to refuse to acknowledge that is silly.
When the question is "Why....?" the answer should not be "How dare you ask?"