Very late to the party of providing feedback - probably too late in reality.
However, it occured to me yesterday, while I was theorycrafting for 7.0, that with the Veteran Ranger being a potential legendary in 7.0, there might finally be merit to the shoulder cannon heal utility.
However, the final state of the PTS had you choose between Guard Cannon / Cryo Grenade / Hold The Line...
This talent tier seems to be extremely unfavoured, since the only real choice to go for is Hold The Line / Hydro Override, when it comes to PvP.
Without an on-demand movement boost and slow purge, Vanguard would essentially be the only class in the game that is unable to disengage from a melee fight at will, or, at the very least, the only class unable to purge a slow from itself. Vanguards are already very squishy when tied up, and if we can no longer deal with being slowed, we will have to turn even more to a playstyle of 'fire and forget' akin to 6.x.
I feel that Hold The Line / Hydro Override should either be baked into the Vanguard kit by default, or at least not compete with Guard Cannon if the Shoulder Missile build is ever to become viable.
Maybe Guard Cannon should be moved to the level 80 tier instead?