For me, the answer is yes. English is pretty lax when it comes to pronunciation since there are so many that speak English as a second language and so we are very used to hearing people mispronounce words. However, this is not true of most languages. In addition, there are languages that contain a much smaller phonetic palette than English and mispronunciation may lead to not only strain for the listener, but to an entirely different meaning to the one intended.

This was a huge problem for me learning Vietnamese, which is a tonal language. The word for beef, father, lover and the verb "to throw away" are all spelled "bo" but with different tonal markings. If you like tongue twisters, practice a few Vietnamese tongue twisters and I promise, you won't be so quick to say you like them anymore. As you know, English is not a tonal language. Instead, it is a stressed syllabic language, so we can have words like read (as in, "every morning I read the paper") and read (yesterday I read a book). These are called heteronyms. Other examples are, "Does the Polish man polish the table?" and "please excuse my excuse." Generally, we are able to understand the meaning because of the context, but to people learning the language, this is one of the most irritating aspects of English. One reason it's so irritating, is that English tends to have very, very long vowel sounds, which is uncommon in many languages. Among my students in Vietnam, vowel shortening was one of the most common persistent problems, which was a big deal, since to get into university abroad they would have to take the TOEFL or IELTS, both of which have speaking components. These mistakes go under "strain for the listener" on both exams.

By learning a bit about the Vietnamese language and reading linguistics texts and EFL teaching methodologies, it quickly became apparent that these problems persisted, because the learners could not hear the difference between their incorrect pronunciation and the correct pronunciation until they had gained sufficient experience with the language. If you think about how children learn language, they begin by babbling, then move on to baby talk. Along the way, many children suffer from mild speech impediments, for example, creating both /th/ sounds, voiced as in "the" and unvoiced as in "think" often reducing the sounds into "duh" and "tink" because they are easier. Through repetition and constant exposure to the correct pronunciation, they often self-correct. This process takes years. Language learners usually don't have the luxury of time to self-correct and so other steps must be taken.

Anyway, pronunciation is important because when people don't pronounce something correctly, or closely to the way you do, it is an immediate signal that they do not belong to the group you belong to. In many situations, this is fine as long as the meaning is conveyed. However, in certain situations it can be very, very detrimental. For example, in business deals. I have seen more than a few deals go sour because an English speaking investor didn't trust that communication was at 100% when dealing with someone who spoke with a heavy accent. It's a small thing, but sometimes it can be poison.

On another scale, consider children once again. How many times have you seen school children make fun of other children for the slightest difference? In my time at school, I saw a few Asian kids get make fun of because of their accents and saw how it destroyed their confidence. These kids tended to stick together and were generally withdrawn. I on the other hand speak with no accent, unless I am very drunk and then I'll revert to a New English accent, which has confused more people than even I expected, but then again, there's not too many brown people where I come from. To top it off, in school I would often mock my white classmates for their heavy, blue collar accents and react to attempt to make fun of my race very, very viciously, often creating cruel nicknames and jokes that stuck with the recipients for years.

What people forget is that language is a technology through which we experience the world. It colors much of our perception even if we are unaware of it. For example, the word "pink." It has many associations in English, like it's a color preferred by girls, it's the color of the interior of rare to medium steaks, it is also a crude way of referring to vaginas. The associations that the word "pink" has are unique to English, though there is some overlap in other languages. Similarly, the way that words are pronounced can convey certain connotations, which the speaker may, or may not be aware of.

There is of course a point when it becomes too extreme, especially in English as there are so many loan words. However, if you are communicating with someone in their native tongue, then yes, every effort should be made to pronounce words correctly. That said, if you encounter non-native speakers making the attempt to communicate with you in English, please try to recognize that they are probably making a sincere effort to pronounce the words as best they can while trying to manage grammar and subject matter as well.

To finally get to your question, I think it can be really offensive to insist on mispronouncing something. It shows a lack of awareness for one and a lack of care for who the speaker is talking to. Ignorance of pronunciation is something else entirely as is the inability to properly pronounce something. Making the effort to pronounce something properly will generally yield a positive response, unless you're one of those people that insists on pronouncing every loan word in English with the pronunciation from the word's parent language. In that case, you're a ridiculous person.