The thing is, people think "it's a huge site I won't be identified" but actually the chances are higher than you think depending on what info you give.
For example, I have twins, and sometimes mention it when I post.
The UK statistics for multiple births are about 19 in every 1000 births. 16 per thousand in England and Wales, less than 2 in Scotland and Ireland.
So if a poster says she has 5 yo twins on a thread, statistically she is far more likely to be identified just because there aren't so many of them .
If she happens to mention over the course of a couple of orher threads that she's 35, lives in Northern Ireland and has a pink VW Beetle she might as well wave hello to all her friends and neighbours. 
Sometimes you can identify someone just because they told the same story on fb as they told on MN. A quick advanced search to confirm and you've accidentally discovered your next door neighbour's marital issues.
Name changing limits the likelihood you'll be identified and limits what anyone can find out about you if they do identify you.
Some people prefer to build up a reputation under one name and be known and that's fine too but there is a risk in that.