Re: Significance
"If it's another variable or saved register contents, then the previous contents will be lost, and/or some unpredictable behaviour might happen when the variable is used"
Yes, but: doesn't matter, got EIP. You've got control of execution and your own code in memory: spawn a new process to do evil things and watch the victim burn. In a nutshell. It's not quite that trivial. Ideally, you don't want the attacked process to crash (especially if targeting an OS kernel) but it's not the end of the world. For you, anyway.
But, anyone weaponising this particular flaw will need to jump through the usual hoops of buffer overflow exploitation. Your payload may well be in a non-executable memory area so the processor will fault if it and the OS support NX, for example. You'll probably blow away the stack cookie too, if it's there - it's not in this program, though.
And the browser or corporate web proxy may reject or otherwise screw up a stupidly long URL so you can't be too loose with your payload.
However, the goal isn't to pwn everyone using Foxit, just enough of those using the PDF plugin on susceptible systems to make it worthwhile.
See others who have pasted the smashing the stack paper; you guys beat me to it.
C.