Of course if you have some strong knowledge/read on this opponent then you might consider folding. You said:
My read on John is something that's improved 10 fold since we first started playing. Realistically I have the worst flush possible vs him and knowing his game he'd 3bet pre with a hand A10+, the way the hand played out, seeing previous hands he's played etc he's pretty polarised imo. So he's either got dry Kd or we're beat.
He's NEVER EVER EVER value betting with a hand worse than mine on the river. We could play this hand 1m times and if he's got a set he's c-raise/calling the flop.
Well that is the sort of read that
might allow you to find a fold with your hand. Maybe. But two things to note. First, this is the sort of stuff that noone else can advise you on (apart from others who know this opponent well too), since it is all based on a personal read. Second, you must be very careful about just how much you modify your play based on reads. You are getting insanely good pot odds given his small river donk and so only need to be winning ~27% of the time to make this a break even call. Weak players take strange lines all the time, and they often do turn up with hands that surprise you even when you think you have a strong line on their play. It is a very bold assumption of you to basically state that he is either bluffing with the bare Kd or has a bigger flush - there will almost always be
some chance that he has a hand that ends up surprising you.
When it comes down to it, this is a pretty easy call based purely on the fact that the pot odds are so good. Do you expect to win this pot? No, of course not... you
expect to be beaten more often than you win. But this is not important, because of the price you are getting. As long as you will win this pot more than 27% of the time then you have a profitable call, and folding would be a mistake.
This is why Sean's comment...
in game i probably call and would not be surprised to be shown better - this has to be a leak.
... is not the right way of thinking about things. We should very often call on the river expecting to lose the pot more often than we win it. The reason for this is the price we are getting on the call. Imagine you take a bird's eye view of all your river calls throughout your poker career; if you see that you are winning the pot 50% of the time when you call
then you are folding far too much! This is a leak.
In my original post ITT I said that when weak players take this line it often means they flopped a very strong hand. I also said I'd still call his river donk, even though I expected to lose more than half the time. Because I would still expect to win more than 27% of the time.
As regards what Lil'Dave says about raising the river... this is not profitable either IMO. Given that my default read on this line from weak players is that we are losing to his river donking range a little more than half the time, it is not possible to consider raise for value. But note that this is based on
my default read, which may be wrong. If you feel that you are still ahead of his range then you can
consider whether or not you are able to raise for value. And if you determine that you figure to be ahead over 50% of the time
when your raise is called then you have a profitable value raise.
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