This is just asking for trouble.
I dunno about other tanks, but I HATE this with a purple passion.
Your #1 job is to HEAL. IF you have EXTRA time, THEN you can throw out a little DPS. Letting me go way down to 20% wondering if I'm going to get an incoming heal or not is not doing your job whatsoever, especially not in Stormblood content where you can have random bouts of 30%+ HP damage, especially on those larger pulls that everybody is bugging me to do.
Sometimes I might not see that stupid AoE marker because of all the mobs piled up ontop of it. Sometimes I have a split second lag that causes me to eat something that on my screen it looks like I was well outside the range of before the mob was finished casting.
If anything like this happens when I am 20%, I am dead plain and simple and you would have failed your job as a healer.
And what if lag happens and your heal doesn't get off as soon as you would like and you're a second too late? I've seen incoming heals when I was 500-1k HP (as a 41k+ tank!) in some Stormblood dungeons. I'm like "Hello? Heal?" and a split second before I drop dead, I get healed..... barely enough to take the next hit. Then the healer has to spam heals to get me back up to at least half and I end up spamming Clemency because I do NOT like being that low.
But if I'm spamming Clemency, that means less MP for Flash or Holy Spirit, that means I can't use Requiescat for extra DPS (because that is reliant upon MP) and it just causes all sorts of problems.
And, not to mention, heal-bombing the tank is a lot of spike threat instead of applying that threat gradually by keeping the tank in the 50-75% range like you're supposed to. It just isn't a good idea.
As a healer, I would never do this whatsoever. Yes, I put out less damage, but tanks never have to feel nervous when I'm healing, and to be honest, the runs aren't THAT much slower if I keep a tank 50-75%.
EDIT: BTW, I get a LOT of commendations when I play Healer. I guess this might be why...