If your job is to write text that is then published, i.e. intended to be read by a large audience, then your spelling and grammar should be good, or you'll be judged by it.
But if you hire someone for such a position - then you *should* look at their english-grades (or other demonstrations of written-communications-skill). It does not make sense to say: This person is bad in physics because he'll sometimes misspell light as ligth.
Your grade in physics, should reflect your demonstrated skill in physics. Language is a nessecary component of that, because it's what you use to demonstrate skill. But other than that, it should not matter.
Consider the case of the foreign student.
Should I get a poorer grade in *every* subject I take in a US-university, merely as a result of my english being worse than the average of natives ?
If my english is poor - give me a poor grade in *english*. Let employers who care about english-skills, judge me on that basis. (maybe that's most of them, maybe not - it depends on many things, such as where I apply for a job!)
But don't tell them: "this guy sucks in math", when that isn't true.
Language is location-dependant anyway. If I apply for a job in USA, my english would be rated as sub-average. If I apply for a job here in Norway, my english is considered substantially better than average.