Seriously, also as a hiring manager I can't imagine not writing a personalized cover letter that's customized to the company I'm applying to. It's literally the first impression that a company has of you as a candidate, and I want them to think 1) I want to be hired 2) that I am thoughtful and prepared 3) that I am a great candidate for the role and company (which, if you cannot explain persuasively in the cover letter, is a bad sign).