It's a matter of credibility and reputation. When someone who has a track record of delivering value to a given organization makes a little error, it's not a bi

It's a matter of credibility and reputation. When someone who has a track record of delivering value to a given organization makes a little error, it's not a big deal, because it's balanced against "karma" that they have already banked. On the other hand, a cover letter and resume (aka "sales letter" for your services) is your first introduction to the organization. Same thing with a vendor proposal. Such a document probably arrives in a pile of several (perhaps even tens or hundreds) of others. In such a situation, and in the absence of personal connections to the decision makers, your credibility is pretty much zero. Your written word is your reputation. In such a case, people are looking for reasons to filter, and being perfectionistic about grammar is a reasonably inoffensive axis to use for a preliminary cut.