Drain + Morphs have always had some limitations that made it not really viable, but the recent patch seems to have made it worse. It is really not worth keeping on the hotbar the way it currently works.

1) It is not usable at all on group boss(2-bracket or more) enemies.
2) It is not usable at on a significant number of single bracket (solo-boss) enemies.
3) It is, as of update 6, not usable if the enemy has recently been stunned, knocked down etc.

This ability needs to be split into stages much like other similar abilities.

For example Crystal Shards from Dark Magic does the same 3 things Drain does but is always usable.
Crystal Shards:
Damage will always apply, even if the enemy is immune to the CC (knock down) component.
The Heal component you can get by spending skill points also will always work, its not a once per enemy thing.
The Knockdown can be blocked by CC immunity without blocking the whole ability.

I propose the Drain skill be setup the same way.

The Damage should always work regardless of enemy type, CC resist, heal component timer etc.
The Heal component should either always work, or work based on a per enemy timer (i.e. once every 10 seconds on a particular enemy). Other players using the skill should not block eachother from also using it.
The Stun should work on non-immune enemies but that immunity should not prevent the damage or heal from working.

This would make Drain a viable skill to have on the bar even during group encounters.