Grammar is absolutely relevant in science, math and various other fields. Noone wants to have to parse ungrammatical documentation or scientific papers.
It's certainly possible to get by in STEM with poor grammar, but it is harder to excel that way. Grammar below a certain threshold impedes natural language communication, which is important even in technical fields.
How well would you consider yourself to speak Spanish: "very well", "well", "not well" or "not at all"? Your English writing is perfect, and I assume you speak English just as well.
> Same with bad grammar sounding less intelligent, it's not actually any less intelligent as long as it still communicates the message.
Where do you draw the line, though? For example, using "bad" instead of "poor" seems acceptable and not an indication of low linguistic competence.
However, consider the sentence "dum talk no mean u dum". It communicates the message, but would you consider someone who said that to be as intelligence as someone phrased it "Same with bad grammar sounding less intelligent, it's not actually any less intelligent as long as it still communicates the message"? I wouldn't.
There is a line between being a snob and being someone who expects something above bare-minimum competence. I can understand the meaning of phrases like "dum talk no make u dum", but I find such speech unacceptable.
> It's funny when Americans claim that British people just 'sound smarter' when British people are just that: British.
That situation is indeed funny, but I think it's beneficial overall. Having a prestige dialect discourages the development of parochial dialects, and helps unify speakers of the language across different regions.
It's the same within the United States. My parents are both monolingual English speakers, but have heavy regional accents. My wife and I both have standard American ("CNN English") accents, and occasionally make fun of their accents between the two of us. As American English is to British English, so is regional English to standard American English.
We recently moved to another region of the country. Noone here speaks our parents' regional accent, and most of the locals also speak our standard American accent. I think we've met new friends easier than if we both had our parent's strong regional accent.
Regarding fluency, bilinguals and monolinguals often have often have similar total vocabulary sizes, but bilinguals tend to have a significantly smaller vocabulary in each language.[1, 2] In other words, vocabulary size(L1, monolingual) ~= size(L1 + L2, bilingual), and size(L1, monolingual) < size(L1, bilingual).