Imagine this:
You're fighting Slooms and they inflict Sleep on you.
A not-so-conspicuous Lumber decides to show up.
*slam*
But you don't wake up. It's not like Freeze where you're broken out, oh no. You're staying there.
*slam*
Rinse and repeat.
You're overestimating how much the Sleep actually heals you; realistically it's quite a small amount relative to the huge damage output of Tier 3 and even Tier 2 monsters. If you were put to sleep, it most likely spells death in all cases.
That's what I've always heard - Sleep is broken.
Not really "broken", it works how it's supposed to, but the status itself is just bad.
For example, you can just let them hit you while using an Ironmight Plate set and the Slooms will deal little damage, but actually heal you with the status.
And Sloombargos could just keep you in place forever if you're soloing and end up in their mist. They'll just keep re-inflicting the sleep status over and over until you die.
It's just broken. I know many people (including me) would like to see them back in the Clockworks, but first a fix has to be made.