One advantage in using relatively soft steel is that it will stretch and bend, rather than crack or shatter, most of the time. This is a general, not an absolute, rule.
That's more of a one-time overload rule. Nearly all metals will stretch and/or work harden, which embrittles and weakens the material, results in tearing or stress cracking. The simple way to think of it is bending a piece of mild steel. Yes, it will deform, where a really hard alloy would snap (at a much higher load, mind you). You can bend it back, but it will be weaker at the stress point. Keep doing it, eventually it will snap. This is the result of exceeding the yield strength of the material; the point at which it deforms permanently.
When examining the mechanical properties of alloys, many folks' first (and perhaps only) inclination is to compare UTS. That is, ultimate tensile strength. It's an important figure, but the yield strength is moreso, since, again, that is the point at which the material will permanently deform. Mild steels tend to have a much lower yield strength relative to their UTS as compared to treated medium or high carbon steels. Let's take a look at A36 low carbon steel, about the most common consumer material. It has a UTS of 60-80 KSI, but the yield strength is only 36 KSI. That means that if you have .25 sq. in. bearing surface (remember, that'd be 1/2" x 1/2", not 1/4" x 1/4"), 9K lbs of bolt thrust will permanently deform it, even though it shouldn't shear on the first shot. Virtually all common medium bore bottle neck cartridges exceed that force, the .308 win running about 11K lbs. Meanwhile, 4140 steel with a 1,000°F temper has a yield strength of 140 KSI, about 4 times as much, which could handle even the 28K lbs of thrust generated by the .50 BMG on that same .25 sq in bearing surface. Your 1045 alloy in the cold drawn state has about twice the strength of A36. Quenched & tempered, it actually exceeds 4140 by a fair amount.
That's one of the reasons I like 6/4 Ti; in the annealed state, it has nearly the same yield strength as tempered 4140. Not easy to work with, though, and does have some drawbacks.
7075-T651 aluminum is another go-to material for me, but it doesn't have the fatigue strength or fracture toughness of steels with similar yield strength (70 KSI), so it's applications are more limited.
Metallurgy is a very complex subject, and I'm by no means an expert. But I do know enough to tell people that they need to carefully research the materials they are considering using for any project, and especially one that is containing explosive force right next to or in front of your face. There's more than one way to skin a cat, so to speak, and you can certainly over-build on account of questionable or unknown materials, but some should just be avoided entirely. Black pipe for shotgun barrels, for example, is just plain dangerous, as is using brake line for .22 rimfire.
The information is out there, and the math isn't that difficult. Yes, many materials require heat treatment/tempering to achieve optimal mechanical properties, but some are available pre-treated, and often actually machine easier that way. 4140HT is much less stringy and sticky than normalized or annealed 4140. If you want to take on heat treating yourself, remember that some materials are more forgiving than others. Sometimes you can get away with torches/forges using the color method, others really need a high accuracy oven and a good, long soak.
For any of you who are interested in taking up this hobby, I would say forget about hardware store or box store steel. It's pretty much all A36, maybe some 1018 CR, and it's ridiculously overpriced. In the small quantities you'll need, shop online. eBay is a good source for best price, but onlinemetals.com and a number of others are probably your best bet if you need specific alloys in specific sizes. Everything you could want is literally at your fingertips, including exotics like various Ti grades, martensitic stainless, Inconel, and so on. And the same rule applies to fasteners; won't do you any good to use tempered chromoly steels for breech parts if a crappy gr. 3 zinc plated bolt is all that's holding it together. I recommend hardened pins or spring tempered music wire in properly reamed holes and, once again, make sure the shear strength of the material at a given size is up to the task!
ETA:
Here is a good source for bolt thrust of various cartridges:
http://www.orions-hammer.com/blowback/
And this is a great one for researching material properties:
http://www.matweb.com/