Why does anyone need 5 names? We've done holiday names on here and all changed after Jeffrey, but one for normal posting and one for "is that you UCM? Embarrassing type stuff" is plenty.
Not really.
Say you post about something that happened in your child's school, or in your office. If any of the school parents or colleagues is on here, s/he will instantly recognise the school or the company, if the event is current. A bit of snooping in your history, and you can likely be recognised (because you spoke of your dogs, or your car, or your hobby). As an outsider it's not enough, but for someone closer to you it will be obvious.
It is safer to name change often if you want to stay private. Most people me included give too many details over time (breed of dogs, number of children, colour of school uniform, job description etc..). Nothing is identifying by itself, but when you link it together it is. It's not fair on your children if you write about them.
It does not help when you ask mumsnet to delete a thread because you have been recognised, and the deletion messages clearly states "poster has been exposed in real life"! Oh thanks, no it's very clear it "was" you, thanks for that MNHQ! 