It's probably not the spelling and grammar in themselves that matter - it's attention to detail.
Any aerospace company, regardless of role, needs employees - pilots, engineers, launch controllers!, whatever, with an obsessive attention to detail. That way expensive and potentially life threatening mistakes hopefully don't get made. If you can't apply that level of rigour when applying for a job, it's a fair assumption that you may not in something safety critical.
And even when posting on Pprune or sending an Email to a colleague, people will make a similar judgement I'm afraid. I certainly have been questioned or complemented before now on my Pprune posts by some quite senior people within my industry (anonymity is not what it once was!), and I know of others who have had similar ' issues ' - ask Whirlybird for example about her Helicopter AFI course and the fact that she discussed it on Pprune.
So, attention to detail is everything, and if you always act as if you were being interviewed for a job you may do yourself some favours. After a while it gets easier - but it may interest you to know that the RAF considers it important enough that they have a self-study course (called ISS, or Independent Study School) to train officers, and in particular pilots, in grammar, spelling and clear writing.
Ultimately working in aerospace is about obsessive attention to every detail.
G