Chiming in as a frontpager, and directing this at Lassthorsemen, yes, deleting diaries is bad form. As the Frontpager Duties page says:

Diary deletion

Sometimes users may feel strongly offended by a discussion on ET, or have second thoughts about posting a diary after a controversy developed in its comment threads. It has happened that in the heat of anger, a user has deleted his/her own diary.

However, by deleting a diary, the diarist doesn't just delete her/his own content: the comments of the diary will be lost, too. To prevent the deletion of more diaries along with the respective comment threads, any frontpager at hand can intervene to withdraw that diarist's right to delete own diaries. This measure only intends to protect others' comments, no negative repercussions follow to the diary-deleting user.

For the record, thus far there have been only two users who had to be stopped deleting own diaries.

In this case, it may be that unbeknowst to me, someone else already discussed this with Lasthorseman in email and the diary was born as a result. It also may be that Lasthorseman deleted it not due to controversy generated but private content, which may make it a borderline case -- can't tell though, haven't read that diary before deletion. (In such cases, a more proper thing to do would be to edit the diary by deleting such content from it, so that other people's comments won't be deleted.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}