Re: Agents that don't respond to your submissions
I'm joining this discussion late (new here, hi!), and I'm not an agent, but I have been in similar situations, on and off, as an editor for a publishing company and as someone who has done on-line recruitment.
First, for our small press, we do send an email to all rejected authors (it's a form email, which I feel bad about, but I guess that's better than nothing). We can do that because we publish in series, so we can send all the acceptances and rejections for one series quickly and easily. Having to reply on a constant basis would be mind-numbing and a real distraction for a basis. Remember that although writing a quick email might take ten seconds (plus another ten to check for errors), that kind of thing can really break your concentration and suck quite a lot of time out of your day.
Second, when I was recruiting people on-line, I fully intended to reply to everyone I received a contact from (and even specified that I would in my email). I got about 250 responses, most of which weren't even close to what I needed. I did start replying to everyone, first with a personal email, then with a form one, but it's really a soul destroying activity. I gave up after about 100 or so I'd guess, always meaning to continue, but never doing so.
So, I can kind of empathise with agents who don't reply. Sending quick emails seems like something that should be really easy, but if you have to do this thing day in and day out, it's surprisingly time consuming and draining. I suspect that every agent who doesn't reply started their career thinking, "I'm not going to be one of those rude agents who doesn't get back to people", before being quickly battered down by the reality...