I discard any employer that requires a cover letter. Cover letters don't tell you anything. If you think they do, you don't know how most people write them, and you probably miss a lot of good candidates.
It's an unreasonable demand. You're not the only place that I am applying. Either I send all fifteen of you a generic cover letter (which is likely no better than not sending one), or I customize it for each of you, which probably takes about another hour per application. It takes you 5 minutes to read.
If you like my application, I have to cut time out of my work day for everyone at your company to call me back, usually at least three times (recruiter initial contact, manager contact, technical phone screen). After that, I take a couple of my paid vacation days off work for on-site interviews. I have to make arrangements for someone else to look after the kids because I'm out of town overnight and the wife does shift work. I have to work a few more weeks to replenish those vacation days if it doesn't work out. All the while, I still have timelines and can't tip my current manager off that I'm looking elsewhere. Again, multiply this by a few companies - maybe three or so get to that point. That's costly, but necessary and understandable. At that point, there is serious consideration for a working relationship, so such demands on a candidate's time are more reasonable. And don't forget, at least one of you likely has some sort of programming assignment or problem set that you use after the phone screen, which I will spend several hours on.
To top it off, I'm going to guess that you don't even have the decency to post salary range in your ads. And, your recruiter is going to try and ask me about my salary history, but fail to mention the budget for the position, and my first indication won't come until there is an offer letter.
So, anyone who does this, thanks for self-selecting!