I think something like Fallout Online could benefit from both First and third person.
Of course the practicality of such things depend heavily on the style of combat the game will have, if it's aiming to have a heavily twitch based combat system first person will have it's merits over third person when using ranged weaponary, where as Third person can be useful for more situational awareness (and shallow ego pumping by admiring your awesome character of course
).
Good example of a MMO using both along side each other would be Fallen Earth, where third person is useful for general navigation/exploration/Hand-to-hand combat, whilst if you're using pistols or rifles you switch to first person for precise aiming.
And to aid the transition of the two, you can always include 'cursor mode' and 'combat mode' and then let the player choose the camera positioning for each separate interaction mode.