Original I-94 is the most important
Its true that when we extend the Visa while staying in the US the new I-94 contains the same # as the original one that was given at the time of entry.
However it does not mean that you can get rid of the original I-94 after getting the new one. In my experience the original I-94 is always needed where ever you have to show your I-94. This includes renewing the H1, transfer of H1, H4 for spouse, Labor application, 485 application, drivers licence. All of these require a copy of the I-94 to be submitted. And even if you may have any number of I-94's before you leave the country again, the original one when you entered the country is the most valid one. You would have to submit a copy of the original always.
I may be wrong but I am of the view that while leaving if you submit the original I-94 you should be okay. The INS records will show that this I-94's validity has been extended when the Visa was renewed/extended.
Its true that there is hardly any clear cut rule written anywhere about all this. But one thing is sure, this small piece of paper even though looks like a piece of trash is the most important paper while you are in US.
And once again its not the responsibility of any of the officials at the port of exit to ask for your I-94 when you leave, its your personal responsibility that you make sure you hand over this piece of paper to them. I have not seen this rule written anywhere myself but has been drilled into me for a long time. Think about it, why would anyone (INS/Airlines) care when you are leaving out of the country, they only care when you enter.