I've seen several discussions around punishment degenerate into what is "real" and what isn't. Whether there is punishment in a relationship, and if so then how it is handled, is a very personal thing.
I am aware of a few D/s relationships where there is no punishment. She is submissive so she submits. If she doesn't do as told then they talk about it. If it happens too much then the relationship isn't working out. The underlying theme is that if she is submissive then she should want to do as told, if she isn't doing as she's told then they need to figure out what happened so it won't happen again. There is no punishment factor, just discussion about what is and is not working. It works for them. It's "real" for them. I get that punishment is not necessary in order to make a power exchange relationship a reality.
In my experience, most D/s relationships do have a punishment aspect, though. Parameters are agreed upon for punishment during the initial negotiations, and then the Top makes decisions based on those boundaries. Some couples keep punishment to impact tools -- flogs and paddles and whips and canes. Others use things like corner time, writing lines, not being allowed to sleep in the bed, etc. Some borrow from the Victorians and use enemas. I'm aware of one particular 24/7 TPE relationship where food choices are used - bland oatmeal is a punishment, steak is a reward.
Where impact tools are used, the recurring question is... if she likes pain then is it really punishment? Ummm, yes. It is. I may enjoy pain once I've been warmed up, but full strength strokes without a warm up are not my cup of tea. It is true that some people do the same things that the bottom enjoys for both play and punishment. For the submissive, since it is punishment it isn't fun, even though the same stroke would be fun if put into that context. If that works for them then great - super -- there are no rules to this, whatever works for the couple is fine as long as it is consensual.
Most of the long term relationships I'm aware of in real life have some sort of agreement that takes away the safeword for punishment. This is only done once the couple know each other really well, and the submissive is comfortable that the Dom knows her well enough to understand the edge between more-than-play and too-fucking-far. And, when the Dom is sure that he knows where that edge is, too. Many couples actually write out what can be done without a safeword - X amounts of strikes with a belt, Y amount of strokes with the wooden spoon, etc. The Dom may choose to do less, but if he does more then the submissive can safeword to stop it.
For some, punishment between two adults is silly. For others, it cements the D/s part of the relationship. Someone is in charge, someone has given up control -- what better way to hammer that point home than for the person in charge to be able to punish the person who has given up control?
Do punishments work for you in fiction? Are you more turned on by the Dom who enforces rules in a no-nonsense way, or by the Dom who is lenient when passing out punishment?