Well lets take any tank support with max level around 5k HPs. In case of Cho, Sion or such, he can spread the FoL to whole enemy team instead of just one so that is 5x50x2 = 500 HP healing. Even to single target its 100Hps. Now comparing that to your Kayles, uh, pathetic heals as you got no AP to give good base level heals, I would also walk over you and your supported friend.
Like said, you need HPs to get it working or some other combo. With other combo I mean Soraka as I haven't found any other champion that can could use FoL to any usable amount of healing.
Soraka with
is pushing 150 AP and 1855+300+200= 2355HPs.
Base FoL would be 28 that is doubled becasue of 4 sec duration to 56. That 56 would in turn be increased by 8+8+5=21% to 84 HPs. Your 'max healling power' makes arguably 44% better job.
Execpt.
Hitting enemy with Starcall will mark them and because of the Athene+Liandry combo, hitting naked lvl 18 Gnar that has 1615 max HP would mean of around 2% of targets HPs worth to be transferd as Blood charges (32) that in turn would be used to boost the FoL healing, also enjoying the 21% increase of healing, thus 38,72 healing done addition to FoL. That pushes the 84 to 122 that is same what the 'healer' kayle build would offer with one less item.
Except.
Starcall can hit multiple enemies thus spreading FoL to multiple enemies with multiple enemies affected with Torment meaning five times more blood charges meaning 32*5=160 Blood charges generated that can be used for healing, again enjoying the 21% increase of healing thus 193, pushing the FoL healing from 56 to 249. That is 205% more healing done with FoL proc than what the Kayle build was offering.
With one less item.
That said, if the rune would heal both champions, the one procing and the one marking, it would actually have some value to others than Soraka.
Now 100-150 HPs, or even 250 with perfect Soraka situation, is one basic attacks worth of HPs from someone that has zero damage builded. That is done over four seconds time while everyone can surely make more than one basic attack in that time, even Graves would do five. FoL can't even heal one super minions hit (even if it would proc from it) and then we have Demolish for a option that would do minimum of 660 damage on towers.