Actually, while I would have agreed with this in the past, I think that that the saying is no longer entirely
true. I think the saying should be modified to say:
If you can't write something nice, don't write anything at all ... unless you can encrypt it.
I think a lot of people find that writing down how they feel helps them clear their thoughts and helps sets things in perspective. Why else has diary writing and todays web diaries/forums/logs historically been so popular? However, I also think that a lot of people don't like writing down their thoughts for the very reasons that Whywait? mentioned.
After all, who would want to write something private which has a very real possibility of being accidentally revealed.
However, in todays world, the use of encryption technologies has made our private ramblings that much more secure. Use something like PGP, and your written feelings and emotions are safe from all but the most determined hacker. I for one have started keeping a diary of my feelings and thoughts and all my ramblings are kept encrypted and secure. At least this way, I can offload my agitated mind into a text editor, without adding additional agitation that someone may read my diary.