As IP/MPLS is deployed in service provider networks, the associated services are increasingly rich and complex: layer 2/3 VPN, leased-line replacement, legacy service migration via PWE3, and so on. The management plane is the means by which these services are managed, which makes it an increasingly valuable information resource. Security of the MP is critical, and this is reflected in the comparatively large number of security schemes available. Just as with standards, you can pick your favorite!
At the very least, a security mechanism should provide five major items:
The ability to use different security schemes; for example, changing from DES to 3DES (privacy) and from MD5 to SHA-1 (authentication)
The ability to partition managed objects so user X has access but user Y doesn't
SNMPv3 fulfils this baseline set of requirements in a standard framework.