TCP/IP CRC question

[Jorge Mendez Bonini <jlmb () cableonda net>]

I've been reading about the TCP/IP protocol stack (TCP/Ilustrated Vol1 
by Richard Stevens) lately and since Ethernet is quite common nowadays 
almost all link layer examples refer to it (I also checked Douglas Comer 
 Internetworking with TCP/IP vol1).
I tried searching RFC but found it very time consuming without good 
results (maybe I'm not used to it...yet)

The Ethernet Frame contains a field known as FCS which contains a CRC.
Now, my question is:
If the CRC is generated from DATA field among others, What's the point 
of using checksums on the upper protocolos (IP checksums etc..)?

Thanks for your time.