Cleanse against plague
They're all M1 gamers used to relying on Thanatophobia and only having to land one hit. A lot of plagues aren't even good with her real power since they've never really had to use it before. And with the recent changes plague can have a decent stock of fountains even if no one cleanses so why not? It's a free heal
Comments
By giving the plague more fountains to activate her power, you're giving her more map presence to end chases faster, and wasting more time than she gains by shortening chases. Cleansing vs the plague is about placement and situation.
If you have an area of the map where there are no gens that need to be done (like the corner of a larger map where the gens are already finished,) that would be a good place to cleanse in terms of positioning. Cleansing right in the middle of a 3 gen scenario would be an absolutely awful place and would give her a massive advantage, while the former would have her need to waste time going to the fountain, then waste time coming back.
In regards to situation, if there is only one gen left, if you know that she is mid chase on the opposite side of the map, if you're on death hook, etc those are all times where cleansing would be fine. Cleansing when in her TR is pretty universally a bad move.
As for Thana, the debuff is a joke. When you do the math for leaving to heal vs staying on the gen, the numbers aren't really in your favor if you can avoid going down instantly when you're spotted. When someone cleanses, it gives her the opportunity to get that instadown if they cleansed and you didn't. The more fountains people have been using, the more of the map is littered with these "kill zones."
It used to be that never cleansing was the better strategy, but now that the plague can force people to consider its pros and cons, its made her a much more fun killer to play as and against.