Heh, we've never accepted print submissions at Twelfth Planet Press - probably for the same reason that I've never owned a cheque book. It kinda strikes me as odd to be publishing science fiction and not be adept with using the science facts of today. Kinda like going to a doctor and finding out that they don't keep up with current medical journals.
Something that I don't think has really been thrown around is the idea that having electronic submissions might improve your slush pile. oldcharliebrown mentions it but I thought I'd take it a step further. The top notch writers don't submit to open call submissions - they don't need to. When you get to a certain point in your career, you get approached and asked to submit to so many projects that you just physically don't have time to write to open calls for works. Probably that's still not the case for "the big three" where it's still a pretty big deal to get published. And that's probably why for them, the argument falls over.
But for the rest of us ... writers need to build their careers. They need to do this by gaining publications and audience exposure. They can't afford to submit a story and not find out if it's made a sale for a year. Or worse, longer. A story that is a year older when it comes back is no longer good currency. Presumably, whilst that story has been making its way to the publisher, maybe by mail, waiting to be collected, sitting in a pile to be read and then rejected, the writer has been working on new works. Works that will be a year further along in the writer's development of their skills. Works that might have already been sold and published. What do you do with the year-old work? Do you submit it and hope noone notices that it's out of synch with your style and proficiency? Do you rework it?
Writers will often tell me that a particular story I read is old. That they want me to keep that in mind when I review it. But the truth is, the story is as "old" as its publication date, not when the writer actually wrote it. And it will be critiqued that way too. The very last thing a writer wants to be doing is trying to shop around stories that no longer reflect their style, skill and proficiency.
And that's why ambitious writers who are actively navigating their careers, building up a publications history, working with as many editors as they can to gain skills, develop themselves and grow professionally, are going to be more attracted to markets that accept electronic submissions and have higher response rates. This can reduce that first stage in a writing career by years (Which has its own repercussions). So too, I think writers who want to stay current, want to be read regularly and often and maintain exposure, either to pitch themselves as a novelist or to maintain their status, will use the same submission tack.
Electronic submissions and quality of submissions are unrelated.