Why is referring to an OP's previous posts considered A Bad Thing?

It's not necessarily a bad thing but people generally put the info they want into consideration in the particular thread. Similarly, people might change odd details about themselves to remain anonymous so it doesn't always help.

Also, people on mumsnet seemed to get so sidetracked. You ask about something, they pick up on some small detail in the post then all of a sudden the whole thread is side tracked due to one sentence that maybe wasn't formed properly, so add to that some thread from a year ago and the whole thing is derailed.

Mainly I think it shows a bit of an over investment. I might see a thread and comment about it but I'm not going to go through the ops old posts because really I don't actually care that much. I'll take the information that's presented to me and not think much more into it.

“Usually it’s done by someone trying to catch people out.”

This. Even though it’s understandable if people change details to make themselves less recognisable to people they know. Looking up previous posts is a bit mean and a bit creepy.

I reply based on the information in the post. I can’t be bothered to go sleuthing for extra details.

Usually it’s done by someone trying to catch people out. Someone did it to me and I’d said something had happened in a time frame and then about 2 years later a referred to the incident and my time frame was a month different to my previous post and someone was like “ah ha, you’re lying”. Nope, just have a full time job and 3dc so couldn’t remember the exact month the thing happened in more than 2 years earlier.

I’ve never seen it done in a kind/helpful way.

I do sometimes look at previous posts, usually if I think they are a Troll. I had someone do it to me the other day though and found it a bit stalkery so I might not in the future