There have to be a hundred posts out there about how important it is to write well in science, but here's 101. I can't tell you how many students I had as a TA tell me they didn't care about writing because they wanted to be a scientist and how many more were shocked and appalled when I took off points from an assignment for atrocious grammar or spelling. I wasn't crazy about it, but I have my limits that were constantly pushed by the students. Ironically, I'm a horrible speller. However, I know this and make sure to spell-check everything I am writing that might be seen by others. I consider myself a decent writer who is always looking for ways to improve and most often I do that through reading and noticing when someone really gets their point across effectively. I look at how they have structured their point or argument and keep it in the back of my head. What did they say that convinced me and how did they get there? If you can lead the reader along so that they reach your conclusion about a sentence before you spell it out, you've done a good job.
When it comes to manuscripts, I always remark on grammar and spelling though I don't take the time to mark up everything as that is not necessarily the job of the reviewer. The gray area is when it comes to grants. In theory we are supposed to be evaluating the science (and broader impacts in the case of NSF) and not necessarily the ability of the writer to actually write, but they are inseparable. Maybe I get hung up on the writing a bit too much, but I find nothing more distracting than a poorly written grant. I have on my desk a proposal for a project including 6 PIs with a budget in excess of a million dollars and I had to put it down after reading the first two pages because the writing just sucks and it was pissing me off. Is that how you want a reviewer reading through your grant? No. Angry reviewers are bad and if they are angry because your writing is the equivalent of nails on a chalk board how likely are they to think your science is kick ass? Like it or not, your writing is a direct reflection of you as an academic and as much as I try to see through the grammatical train-wreck and missing words in the back of my head I am thinking that if this proposal wasn't worth your time to edit and clarify, why is it worth my time to read and thoughtfully respond to.
So, dear readers, repeat after me - "Both verbal and written communication are essential facets of science and should be skills that are constantly honed, just like the techniques you use in the lab or field (or PLS will send you back the charred and shredded remains of your crappy grant)."