I'm not a teacher, but I agree with you 100%.

I think the same thing applies to most colloquialisms, slang, and idioms. They should be learned, but only as passive vocabulary, not used. 

It's not even a question of giving offense. As with children, we make allowances for foreign speakers. It's just that using nonstandard language does not make a foreign speaker easier to understand, more natural, better, or more "native." Most foreign speakers have a foreign accent that requires a listener to work a little hard to understand them. Listening "through" a foreign accent is easier if what is underneath the accent is predictable standard English. We don't really expect to hear profanity, colloquialisms, slang, and idioms in a foreign accent. It's confusing and it slows things down.

The first goal of a foreign speaker should be comfortable communication. A foreign speaker may think that using "the real English" will make him or her sound more "native" and make communication more comfortable, but it rarely does.

It's very confusing, because a foreign speaker may hear these words all the time in songs, on YouTube, in movies, and find it hard to believe they can be bad. Consider something as seemingly harmless as "poop." In the US, it is very acceptable to talk about "dog poop" or talk about a baby "pooping," and yet it is completely unacceptable to say "Excuse me, I need to go poop." I can't guess how many years of studying and language and living in a country you would need to do to be able to handle this sort of cultural nuance correctly.

I disagree... cursing is a part of the experiences we want to have in the target language, so I'd argue that people after a certain level are to be "allowed" to curse in their second language. To me saying you shouldn't curse in your target language is as unthinkable as saying you shouldn't confess in your second language because you wouldn't "feel the same" or something. I mean, sometimes it could sound unnatural, but after a certain time - and depending on your first and target languages - cursing in the 2nd tongue might be less awkward than cursing on your own. Besides, although there are moments you cannot control and simply, you know, curse, sometimes you do it in order to achieve something (a desired effect), such as offending someone or getting them to do something, or just showing to everyone around how pissed you are etc.

Overall, I think that deciding to curse (or not to) depends on context, but it also does when you are a native speaker, so I wouldn't draw lines between native and non-native for that particular thing.

You'll never be a proficient speaker of British English if you can't throw a mild swear word in when you need to. =)  If you don't eff and blind* down the pub, we will think it a little strange, no matter where you're from.   All of my non-native friends started swearing long before B1 level!

Although I can see Dan's point about unpredictable language being more difficult to understand and basically agree, in most (working-class at least) bits of Britain, mild vulgarity is absolutely predictable, and I'd be surprised if we were the only culture where this was the case.  

*swear a lot

I'm not a teacher, only a person fluent in three language, learning Spanish as my fourth.

I get what you're trying to say by cautioning people. I also feel that it's contradictory with a strategy that is highly recommended by language teachers far and wide, which is to think in the language you're trying to speak. I'm a native Hungarian. If I stub my toe while in English-mode, there is no way I'll stop to think about switching to my native language before the swear word leaves my mouth. 

Transferring a word from short-term memory into long-term and fully integrating it into my vocabulary generally requires hearing it, reading it, saying it and writing it a few times until it sticks. All of this, in context. Doesn't matter whether it's a "good word" or a bad one - the only difference is that the bad ones I usually practice on my own, and this is what I'd recommend to everyone else as well.

Also worth noting that I don't regularly use a wide range of swear words in any of my languages, but things like screwed up, or f-ed up are in my everyday vocabulary. Saying "I'm sorry, I screwed up big time" or "I'm sorry, I made a big mistake" convey a slightly different meaning, and if I mean the former, I will say the former.