I will keep an open-mind on the "giving up" part.
Let me share with you what I have in mind. English has standards, not just one, several. We speak English in our country that's why we don't have much problem understanding English variants. I live in China for more than 7 years now. I work in an American company, day in day out all our e-mail, MSN, most importantly, spoken language during meeting will be English and Mandarin. Mandarin, from all over China, some colleagues speak with a very heavy Sichuan accent. Then English, people in China adopt American accent, since they have more exposure to American media. I manage Europe region, most of the time I listen to English with very heavy Irish accent. Some of my buddies manage Australia, they have to get used to the heavy Australian accent. I grew up in half-English environment, I don't have much problem catching words with slightly different pronunciation. But those Chinese colleagues of mine, they don't speak English at home or anywhere else in the city, except in the company.
Question to the version which an outsider should learn. If he's going to spend most of his time in Penang, like a Dutch author here, why does he need to be so concerned about Taiwan variant or Xiamen variant ? He needs to make sure he has the fundamental ability to communicate, in a local variant, before he embarks to learning other variants. If he's not going to live in a Hokkien-speaking region, he can decide any variant that he wants. Like a Beijing resident, a Taipei resident, are we really convinced that people from both cities speak exactly the same "standard" Mandarin ? I personally, would say, the answer is NO. They just have to learn and absorb from each other. They have a Mandarin background, they speak Mandarin, they just need to fill up the possibly 20% gap. At the end of the day, both of them will be 120% Mandarin speaker because none of them needs to give up what they originally know about anything in their own variant.
I learned French, my teachers were French. Then I met a Canadian friend previously, I found it very difficult to understand Canadian French. Phonetically, some words are not pronounced the same, they have differences in accents, too. Vocabulary, Canada and France both have different ways of saying things. I heard from my teacher before, that he had problem talking to Canadian. Then, he managed to listen to Canadian French, respond in European French.
Personally, I don't consider Hainan or Teochew a "variant" in Hokkien. When one person starts to learn Hokkien, he's going to face Xiamen, Quanzhou, Zhangzhou, Taiwan. I also don't believe that we have so much variances among the 4. If, when he learns Quanzhou and he finds it difficult to understand Zhangzhou, he can ask the speaker to speak more slowly. If he doesn't know how to ask, he's not ready in the language yet.
We hear people speaking the same Cantonese today because everyone tries to learn from Hong Kong TV series, my family is one of them. Personal thought, there's no standard in Cantonese, there's only trend in Cantonese. People pick up words and lines from what they hear. None of my friends actually learns Cantonese by sitting in class. That's why, I encourage people to learn Taiwan variant because that's by far the only region that produces media to help you learn Hokkien. Learn from the variant that you will most likely come in contact with. Like I said, Xiamen variant "may" be the 1st to extinct even though it is the "standard" in China. If a foreigner comes to Malaysia, does he need to worry about Indonesian variant ? Master Malay, then he will be able to start absorbing differences in Indonesian.
I apologise if I sound a little offensive when it comes to this. I love Hokkien, I will help as much as I can, if anyone's about to give up.