So...
"Needing" an action ("need" defined as being able to fire in relative safety without worrying about a bolt blowing back through your forehead) with beefed-up lugs and recesses to handle the bolt thrust of a .338 LM...or a .50 BMG, is to allow for "less than perfect" chamber and headspace clearances?
No...........I can't even see a connection.....??....... Altho "less than perfect" is a nebulous, undefined concept in this case. I have no idea how you're defining this phrase but for ME perfect case fit is a half-thou all-round. I've never met anyone, not one single soul on this planet who defines "perfect" the same way I do. What is your description of "perfect?"
And is "safety" the paramount thing? or "accuracy?" Or both? Or is there a difference in your world between "accurate" and "safe?" Are the two items related? Exclusive? Separate?
Unfortunately, the problem of "safety" isn't so easily encapsulated with one small facet of the industry. As a construction guy I know from lots of experience with missing body parts and dead people that no matter how "safe" you make a thing, some redneck can hotwire it to blow sky-high..... If I were to opine re "why the beefed up lugs on bigger actions" I'd not even include nor consider the items you mention, "chamber and headspace clearances."
I can't for the life of me see how either item CAN BE a safety issue.....
#1, if clearances are TOO LOOSE, the gun won't even fire.
#2, if clearances are "slightly excessive" and the gun fires..... it's safe, nothing untoward can possibly happen.
#3, if clearances are TOO TIGHT you just can't chamber the round. This causes endless bitchiness and grunting on the innernet but no HARM can possibly be done (except "show me on this doll where the innernet hurt you" type harm)
No, IME the reason guns are "proofed" to a standard is to allow for two much more common problems. Both redneck-related. Both "outside the norms" of factory stuff. First has to do with reloading and second with cleaning....
#1, overloads. Guns are proofed to "standards" which vary over time and locality regarding just how badly yer average redneck can screw up in his reloading procedure before he loses the front side of his head to stupidity.
#2, wet chamber. Again, rednekkid stupid sets the standard and rules the day. A large percentage of shooters have developed the ability to read and over time they run acros't the concept of "cleaning the firearm"....... with varying results. But a blessed few truly understand the part where the author speaks about "clean dry chamber." I've watched countless guys wring solvent off their patches until it's dripping out the magazine well. All the old glossy-mags contained articles about how "when glass-bedding for accuracy you must address the oil-soaked wood in the stock" as though it's simply a known fact that when you clean, oil and solvent just runs out over the gunnels!
Crazy....
I quit going to public shooting ranges many yrs ago but remember many many times standing with the "pros" and range rats in "the cleaning area" and never one time did I see Brake Kleen, or Ronsonol or Ether or any sort of chamber drying method employed. I've asked dozens of people "how do you dry your chamber?" And they look at me like I've grown a third head....
Liquid in chambers drives pressures SKY-high for a variety of reasons and most of them increase "bolt thrust" dramatically....
And BTW, to address your original conception of "need?" I guess you called it...... I won't build a 338L on a Rem700 or Savage action. But Savage has. And until one blows up from stupidity they'll continue to do so. As long as sane loads and DRY CHAMBERS are employed they're perfectly safe. And most of the gunkholers who'll drive the market will buy them for the same reason they drive a howling 1-Ton Dodge Dooly Deesel to work down the freeway. And they'll be fired a few times with factory ammo, never cleaned, and all will be GOOD
And "safe"
My way is better. 
