It depends on the genealogist.

I have personally found errors in personal documents (e.g. wrong dates on photographs) and I have found errors in official documents (e.g. US Censuses) when I do genealogical research on my family. With errors in both types of documents, I use them regardless because some information is better than no information. And the more sources to support an event, the more likely that the event information is correct.

hi! while there may be people here with some genealogy experience, it would be worth x-posting to /r/genealogy

You can of course use them, and I actually suggest you do so if you are lucky to have such " documents " on a person. Diaries, letters and the like are not " official documents " indeed but they are incredible things to have since they can allow use to understand something no birth, death, marriage certificate or justice document can do --> have an insight on who the person was.

Let say it is not difficult to learn about your grandparents, great grandparents and if you are luck great great grandparents because there could be people alive who remember them.

For example I know pretty much everything thing there is to know about one of my great great uncle : where he was born, what he studied, what he planned to do with his life, what he did during WW1, his wounds, his heroism action in the trenches leading his men, his death, where he is buried. That's actually a lot but I also have a letter he never sent to the woman he loved and with whom he planned to marry. No official will ever say they loved each other and could have lived together. If I ever find the descendants of this woman, showing them the letter could change a lot on what they know of her since there is no mention of it anywhere. It's not an official document but it helps to get an even better view of the person.

For many genealogists, their ancestors only became a list of names and numbers ( the date ), with their professions often being the only thing you know about them. This can lead genealogy to be boring and the first thing to do to make them " alive " is to learn about the history they may have seen or be part of ( e.g. what happened in their village/town during X war or Y event ). But if you can find even more to it such as a diary,...Etc then never think it is of no use, even if you do a professional research for someone. He actually may even be more happy to see a letter than a list of dates !

As an example, I went to a conference once and the genealogist explained that an Italian trader of the early XVIIIth century has written a diary about his trip in various parts of Europe. This diary is now in some local museum ( can't remember where in Italy ) and has been digitized. In this diary, the Italian traveler mentions that he stayed for a couple of night in a small town in the Jura ( today a French department ). The genealogist's ancestor was the innkeeper and the trader described him in his diary as a small, fat but joyful man who showed a local dance. How lucky can you be to have such a description of an XVIIIth century ancestor ! It's not official why not take it into accounts ?

And as u/itskt says, it also helps. Even if these are not official documents they can still provide even more info on dates,...Etc The reason why most people don't use it is because they don't know such things exist about their ancestor. It also raises another problem of modern genealogy which is not really the point here but I'll mention it : " copy-pasteology ".

Many people simply copy-past what they find on popular ancestry websites, without researching a single thing actually. That leads to the spread of errors about X person ( everyone can make a mistaken, I've done many and big ones ) but then if people simply copy your mistakes without verifying it is problematic. That's why many " genealogists " will never use personal documents, because they did not look for it.

So yes, if you ever find such documents, especially if these are old then don't think about not using them. These are of an incredibly rich value when it comes to knowing the persons you are researching.