Because if the fungibility of Bitcoin is lost then Bitcoin is lost.  See my signature and this:

https://bitcointalk.org/index.php?topic=340272.msg3652657#msg3652657

Also, reading this entire thread:

https://bitcointalk.org/index.php?topic=334316.0

is a very good introduction to the fungibility issue.  There are many very good posts by gmaxwell which explain why this is so important.