Is Mac masquerade necessary ?

Generally you do not need it.

"Ordinarily a failover to a new Active system will result in issuance of Gratuitous ARP broadcasts with the newly Active system’s MAC address supplied as L2 address for each L3 Failover Address (Virtual Addresses/VIPS, Floating Self-IP’s, NAT addresses)"

Some devices ignore Gratuitous ARP's..

So..MAC Masquerading solves these problems by using a common L2 address shared by BIG-IP devices for L3 traffic; no ARP entry is changed, only Switch’s MAC address/port forwarding table.