Here’s the missing half: if there aren’t OA journals in your field, or if your short-sighted promotion and tenure committee narrows your options, then you can publish in conventional journals and still provide OA to your work through self-archiving.
More appropriate for someone in my position is:
As students and young researchers, you may not yet enjoy ultimate control over the journals in which you publish. You may prefer to place your personal advancement over public access to your work — and while the entrenched hierarchy continues to reward this behavior, you’ll meet little opposition. But before you fire off that next manuscript to Cell, consider this: scientific journals exist to record and disseminate the research results, not to make publishers rich or restrict access to vital information.
Of course, I’m in a computer-savvy and web-savvy field, so it’s not so much of an issue. But there should be an ArXiV for other fields as well to help people disseminate faster.