Sorcs can afford to wait this time by stacking other shields to protect it. Other classes who use this skill cannot afford such time. Infact this skill is super powerful on a sorc with low health due to how it scales and that they can protect it almost endlessly it seems.
Magblade are forced to use it as a 'defensive spammable' and sometimes it goes to some rando in a zerg instead of the user causing them to die. They also have to disengage to get the warden to heal them, most of the time they have to deal with the initial hit of the ward and hope the rest of their HoT effects bring them back up
Templars have an option to use it but most do not
Some mDKs use this when taking a lot of heat
Some MagWardens use this due to the tight cone of shrooms. Healing ward is a big help to any Magic Warden that has a hard time aiming their shrooms
Not every class can heal to full in one GCD save for a not defiled templar. Magblades have to fully disengage. DK needs to have had embers going for awhile before they get a good heal but can spam embers a few times to stay afloat