A.There is no disadvantage.
B.It will add 16 bytes per packet of overhead(8 bytes UDP header, and 8 bytes of NON-IKE Marker).
C.It will add 36 bytes per packet of overhead (20 bytes of IP header, 8 bytes of UDP header, 8 bytes of NON-IKE Marker).
D.It will require that both crypto peers support this feature in the IOS version. Both crypto peers must also have this feature enabled.

A.access control and billing can be done on a per-user, rather than a per-site basis
B.supports Link Fragmentation and Interleaving 
C.provides the ability to connect a network of hosts over a simple bridging access device
D.provides a consistent means of authenticating users (RADIUS for example)