Aside very niche situations, AD Yi exists as a three ability champion due to how rarely Meditate is useful.
Here are reasons it isn't very useful on AD Yi:
1. Yi's inherent frailty making him a target and the prevalence of interrupts add up to mean that he is lucky to keep it up for more than 1.5 seconds in a team-fight, and he tends to die the moment he is disabled.
2. The number of nukes that also interrupt channels make blocking damage with the skill highly unreliable.
3. With just a dab of lifesteal, Yi can restore more health by attacking than Meditating. This has the advantage of allowing Yi to still do damage, reposition, and generally not be a squishy sitting still uselessly.
4. AD Yi typically works in a very short time window in teamfights, trying to do as much damage as possible before being CC'd and focused. Meditate uses up precious time without helping against getting CC'd and focused.
To present its few strengths, I'll list some of the rare uses for Meditate on AD Yi:
1. Recovering small amounts of health when there is nothing to lifesteal. On a long cool-down.
2. While dueling the other Melee ADCs, Fiora and Tryndamere, it can help him survive their ultimates. It helps in a similar way against Kayle.
3. Karthus ult.
4. Zilean's Time Bomb.
5. Ultra-rare situations where staying alive two more seconds in a losing duel will mean an ally or turret saves you, but only if your enemy doesn't have an interrupt.
I understand that some found AP Yi's ability to heal in lane made the ability frustrating for some and led to nerfs which have been announced, but Meditate has only been a (relatively) reliable tool for keeping Yi alive in team-fights with AP Yi in its pre-nerf form.
If Yi, a squishy carry who dives deep into fights, cannot use the ability to negate damage if not interrupted, what purpose does the ability serve?
I'm not calling for the ability to be kept the same, I'm asking for it to be reworked entirely if its heal function is seen as toxic where it is powerful enough to be useful.
If Riot wants the ability to help Yi survive longer in team-fights, I'd reccomend altering Meditate to work like a cancel-able Zhonyas.
With this, Yi could dodge any CC with good timing while accomplishing little if the enemy outplays him and saves their CC or uses it when not expected, giving him a skill indexed way to live longer in fights.
To preserve some of the spirit of AP Yi, they could also give such an ability a passive which would heal him on champion kills based on his Ability Power, but that may be superfluous.
If Meditate's heal is considered toxic when it is strong enough to be useful, rework the ability rather than nerf it.
Edited with the arrival of the Patch Notes. My old points still stand: Meditate will remain a pathetic skill on AD Yi and will now be unreliable for any team-fight role as AP Yi.