A I-551 stamp on a blank I-94 is preferrable to an I-551 stamp on passport

[AmericanWannabe]

Because after you get GC then you have two GC. You can carry
your original I-551 and keep the plastic card at home. It is not a big deal
to lose I-94 with an I-551 stamp. But it is abig deal to lose your
passport or the plastic GC

INA 264 does not say it has to be unexpired one.

http://straylight.law.cornell.edu/uscode/html/uscode08/usc_sec_08_00001304----000-.html

INA264 (8 U.S.C. 1304)

(e) Personal possession of registration or receipt card; penalties
Every alien, eighteen years of age and over, shall at all times carry with him and have in his personal possession any certificate of alien registration or alien registration receipt card issued to him pursuant to subsection (d) of this section. Any alien who fails to comply with the provisions of this subsection shall be guilty of a misdemeanor and shall upon conviction for each offense be fined not to exceed $100 or be imprisoned not more than thirty days, or both.

 

[pralay]

The temporary stamp exipires after one year (whether it's on passport or on card).
Why do you think it's big deal to loose GC? You know what, you can do endless permutations and combinations about "loosing" something. In the end, that will give you only sleepless nights. There is no reason to be paranoid. If you loose your wallet, it's too bad because you lost your credit cards, ATM cards, DL and green card. I guess you will get more headache to stop unathorized transactions of your credit card/ATM cards (or the worst, fixing your credit history) than obtaining a new green card.