Even though I personally like Flash (AOE, uses MP rather than TP, can be used during pacification from Berserk, etc) it's not a must have. Provoke, on the other hand, is. Without Provoke you basically literally can't tank Thanatos in CT, EX Primals (even Garuda), or Turn 2 the normal way. Tank swapping mechanics require you to have Provoke and (I assume) there will be more of these in the future.

Provoke is definitely a must, Flash not so much - though by having Provoke you'll also have access to Flash if you need it.