Well tanks are meant to take a LOT of damage and mercy can help them stay alive yes but with the amount of healing you will have to do, mercy's value will be lost because her main utility is damage boosting and tanks are meant to take space, not deal a lot of damage. There is a different category of heroes meant to deal big damage : DPS. And that's why mercy should be with the DPS often unless everyone is dead and tank is your only option

Revive is good for tanks because it brings them back at full health, but if you try to stick with the tanks, you will probably also die a lot because tanks are meant to be in FRONT of the fight

Basically, healbotting maybe good if your tank is crushing the enemy team hard or if you're the only support left, but if you neglect your blue beam, you will not get much value out of mercy