Bold is not sufficient
First off, using bold to indicate headings is just as bad as using headings to indicate emphasis. If a user gets it wrong - either way - then edit their post and fix it, just like you'd do for any other formatting error.
Headings are important
I prefer reading answers that are short and sweet vs. verbose and rambling. That said, I also prefer reading answers that are comprehensive vs. terse and incomplete. If you need three sections, five sub-sections, an introduction and an appendix to provide a good answer, then omitting the headings that denote these sections just robs me, the reader, of important visual cues. You might as well omit paragraph breaks while you're at it, and force me to go through your wall of text with a highlighter.
Drawing attention to your answers by making them attractive is also important
You worry that high-rep users are "taking advantage" of novices by formatting their text in an attention-grabbing manner.
So what? If they're writing and formatting their answers in a way that makes others want to read them? Well, more power to them. If you want to format your answers as a single huge paragraph of superscript, that's your prerogative, but don't expect a reward for your sadism.
Anyone willing to properly use bold, italic, code, heading, list, quote or any other formatting style available to them is helping the site by doing so...
Semantic markup is important
To your average human reader, a heading is nothing more than big, bold text. Except when it's normal-sized bold text, or underlined indented bold text, or bold text with a bit of extra margin after it, or...
Yeah. Headers aren't really about making the text bold.
If Jin [SOIS web designer] decides to style headers in a different way, then the markup allows him to do so consistently across the whole site, with a small change to a stylesheet somewhere.
If you get fed up with over-formatted text and rip your own eyeballs out, the screen-reader you buy will pick up on the headings and let you navigate more effectively.
If SO ever decides to support long, detailed answers by allowing anchor links to sub-sections, headings will provide a simple and elegant means to achieve this.
Meanwhile, that bit of bold text followed by a double-newline does nothing useful.
There are no rules, just guidelines
Well, ok, there are rules. But users who "break" those rules generally don't get anything for their trouble. Your own personal tastes certainly don't constitute a set of hard "rules" that others must live by...
If you don't like the way a post is formatted, then edit it. If you can't edit (or your edits are rejected) then down-vote it. If you don't feel it deserves that, then ignore it, give thanks that SO doesn't support
<marquee>, and get on with your life.