Hello Zaman
The truth is that I spent quite a bit of time reading through various Cisco documents, trying to get a clearer answer for why the VPN label is being used. The truth is that routing could successfully take place without it, as you mention. But when all else fails, talk to Rene or read the RFC 
. So I talked to Rene and he referred me to the RFC. It specifically states:
While this approach could theoretically operate in some situations
using a single level of labels, there are significant advantages in
using a second level of labels that are bound to IPv6 prefixes via
MP-BGP advertisements in accordance with [RFC3107].
This has been taken from page 5 of RFC4798 which describes 6PE, and it further describes the advantages of keeping the VPN label.
I hope this has been as helpful for you as it has been for me! 
Laz