I did say "rarely" - refer to these posts
http://forums.p38forum.com/forums/viewtopic.php?t=3161&highlight=cracked+frame
http://forums.p38forum.com/forums/viewtopic.php?t=6969&highlight=cracked+frame
Further review thru the forum will show there is generally more concern about the early slides cracking than the frames. Not that there are a rampant number of cracked slides. The concern is legitimate but you'll find many more posts where people have used their P-1s extensively, as opposed to experiencing cracked frames or even cracked slides. Especially if you're talking about a gun that when purchased had been rarely, if ever fired.
Note the comment in one of the above posts that the hex-bolt was not installed to prevent the frame from cracking but to reduce wear (read rounding or peening) at a key point. When you disassemble the P-1, you'll see that the hex bolt comes into play with the locking bolt when the slide is returned to battery by the recoil springs and not when the locking bolt strikes the frame from the more violent blow-back forces of the fired round. But if you have a P-1 with the hex-bolt you likely also have all the various improved parts that were added to the gun over the years it was produced
The P1s were used by the German police and were intended for high volume use. Consider too that it's unlikely the police would use for 40 years, a design prone to the frame cracking. Read thru the threads on this THR forum and there are examples of failures with just about every gun ever made. But exceptions don't mean that's the rule. Given that tho, if you're buying a used pistol there is little chance you'll know exactly how a pistol is used. And without accusing anyone of anything - people get advised not to use P+ ammo, do anyway, have problems, but don't want to admit they ignored good advice, and plead woe is me.
Realize that any P-1 you buy at this point is going to be 15 - 50 years old. Replacing your recoil springs may not be necessary but honestly it's cheap insurance to help protect your investment. I have shot about 1500 rounds thru my P-1 and have no qualms about putting another 1500 rounds thru it. (and my gun was an issued weapon with two armory checkup marks on it). It is a fun plinker - would I use it for CC or home defense - not likely because of the ammo limitations - I haven't had good luck shooting anything but ball ammo with it. Also, although mine's fairly accurate it's a military weapon and not super accurate. But I definitely enjoy taking it out and blasing miscellaneous fun targets.