Thanks, Sunic! But explain what you mean by "IP Proofing."
IP stands for Iterative Probability. While it is common knowledge in optimizer circles that luck is the enemy, IP Proofing codifies both the term used to describe it, and the measures used to obtain it.
The principle of IP proofing is simple. Campaigns are long. They will contain combat. PCs must win every combat to win. Team Monster must win a single combat to win. For this reason even small chances to fail in any given fight quickly approach one over the course of a campaign, and if you have a large chance to fail in any given fight, you will die all the fucking time. And because of the skewed victory conditions, a "large chance" can be as low as 10%. In other words, 90% chance to win a fight = your character is more likely than not dead on the seventh fight. Since there are 13 and a third fights a level, and therefore that's only about half of a single level, clearly this is unworkable.
The act of IP proofing a character is the means by which you get that failure rate as low as you possibly can, preferably to the fractional percentage levels. This requires high enough saves to pass level appropriate effects on a 2 or better, immunities to common effects to further protect you, and block the no save, you die effects, means of avoiding full attacks, real defenses (Blink, Mirror Image, miss chances), anti ray defenses (Ring of Entropic Deflection, Ray Deflection, etc) among other things. Rerolls for example, to further mitigate the 1s.
It is not possible to IP proof low level characters. You cannot get your saves high enough to do so, don't really have immunities and abilities, etc. Mid level characters can be IP proofed. High level characters are relatively easy to IP proof.
It is also not possible to IP proof Pathfailure characters. They left saves the same or lower, depending on class, things that boost saves other than the basic, and not good enough by itself cloak are not available, and they had a big thing about whining about immunities, mostly because they could not accept that the game changes fundamentally every few levels. The best you can do is go all casters, spam save or loses, and never let the enemy get a turn. Rocket Tag at its most extreme, because you have no forcefields (read: IP proofing techniques) to block it.
If it were not for this though, it'd work the same way in Pathfailure as the Fast progression is exactly the same as 3.5 leveling. If the DM is an idiot, and let's face it, we're talking Pathfailure, so they likely are he will instead use Medium or Slow. This of course completely fucks over everyone, as you now need even more IP proofing measures to get anywhere. But you can't get them. So feel free to treat it as a bad video game. Not to mention making everything more grindy, slow, and unrewarding. Also something the Pathfailures like way too much.
But back to 3.5. IP proofing is necessary for campaign survival. Even normal power campaigns will quickly rack up a death toll. By IP proofing your character, you both preserve your character so you can continue playing and enjoying them and preserve campaign continuity. Both are obviously desirable. The only other alternatives is that the DM undermines the entire campaign by cheating, or that the entire campaign is undermined by a revolving door of characters. Not the same characters leaving and coming back. Different characters.
For you see, Raise Dead, which I know people are probably wanting to mention by now makes you lose levels and lose money. If it just happens once, this is ignorable. Thing is, once you die once, you're down a level. So it's easier to die again. And now you're a cohort, with substantially nerfed treasure. Which means the whole party, including you is better off if you just make a new character, and you certainly are not going to be able to contribute after that second death.