Now, there are many who believe the quad-rail is the best system to attach your kit to. M-LOK, Keymod, etc... these are things emasculated men put on their AR/M4/m16-type rifle. Picatinny rails work the best! So what if they're heavier, bulkier, harder to grasp, and slower into action? If you're worried about any of that, you must need to lift weights and learn how to be tough... what a pathetic excuse for man!
Of course, that's a little bit of satire. Anyone who thinks Kyle Defoor or Travis Haley are little sissies is pretty much an idiot. But, many people actually do speak about people who use things like Keymod or M-LOK in those terms. Are they right? No. Not at all. They're idiots, like I said. If you're sitting there reading this and have a quad rail on your rifle, fear not! You see, it's not that Picatinny rails have no place or that your gun has become an obsolete piece of trash, destined for the scrap heap. It's certainly not to say you are an idiot if you use a quad-rail. But, if you think Defoor and Haley are sissies, you definitely made an error in judgement.
So, what's wrong with a quad-rail? Well, it's not so much "wrong" as it is "less than optimal." For starters, the weight is not optimal. Quad rails are heavier than these other mounting systems. So, some ask, why not just lift weights to get stronger to compensate? You see, a guy who uses Keymod can work out too and he's all the faster for it. Keymod attachment points are cut-outs in the handguard that actually reduce the weight of the handguard and allow you to add your attachments! This alone is reason to go with a Keymod or M-Lok rail. Also, I totally respect a man who can "embrace the suck" to endure pain and discomfort... even that of grabbing a cheese grater, or otherwise. But, you're deranged (more precisely, a masochist) if you think it's better to be enduring needless pain/discomfort, than to get more ergonomic equipment. Having Quad rails puts you at a disadvantage because of their weight, uncomfortable grip, and bulk. It's as simple as that.
What about the detractors of systems like Keymod and M-LOK? What are their chief arguments? Basically, they complain about how weak those attachment types are. Personally, I've never jumped out of an APC in Fallujah, but I do use my guns... a lot. I can tell you that Keymod and M-LOK will hold up to any normal wear and tear. I can also tell you that you shouldn't be mounting an optic to your handguard... or pretty much anything else that is mission critical. I mean, it's not like your gun becomes inoperable if a "broomstick" or vertical grip comes loose. If real men lift weights to lift heavy rifles, can't those real men shoot a rifle without a mall-ninja-esque vertical grip? These are not exactly mission-critical pieces of kit. (Just in case you're wondering, I do use small vertical grips. Hot barrels and gas blocks make them pretty useful a times, plus they act as a hand stop. But, I would do just fine if mine fell off, especially if I'm wearing gloves, which is my point. I'm just poking fun at the internet commandos.) I can also tell you that the 12 o'clock picatinny rail on your handguard is there for any such attachments that are mission critical.
So, in short, Quad Rails are not needed and add extra weight. If you're looking at a new handguard for your AR15, I would look for a continuous 12 o'clock rail, enough length to allow you reach out near the muzzle to control the rifle better, and a weight of 10oz or less. Virtually all of those are going to be Keymod or M-LOK. If you have a quad rail and really like it for what it is, you don't have to go lighter just because I say so. But, I highly recommend trying out lighter weight options to see how you like it - I bet you don't go back.