The reason for waiting is that hormones being off balance from a miscarriage don't guarantee another miscarriage, but make it FAR more likely. You'll hear a lot about women who go into fertility specialists because they've miscarried 6 times in a year. The fertility specialists have them wait 3-12 mo, and they're pregnant no problem, no intervention needed.
If you were early enough along ... a chemical pregnancy... then the numbers don't change, and you can be pregnant the next month with no increased risk of miscarriage. But if you were late 1st trimester, or anywhere in your 2nd or 3rd... the numbers skyrocket and the risks for miscarrying the next get pretty high unless you wait.
There are 2 general schools of thought that are linked together:
1) Wait a few months so your hormones are evened out
2) Wait until you feel ready (which for many is a sign that hormones have evened out)
The downside to #2 is a) a lot of parents want a baby so badly that 'feeling ready' happens LONG before mom's body is ready, and b) that a series of miscarriages is hard not only mentally/emotionally... but a series of them can mean it can take your body up to an entire year to 'reset'. Impatience in the beginning, making things take a heckuva lot longer that waiting a couple months.
Is another miscarriage guaranteed if you don't wait? Nope. Of course not. :) LOTS of women miscarry, get pregnant immediately, and have babies. Is it more likely that you'll miscarry or have a chemical pregnancy if you don't wait for the chemicals to settle down? Yup.