I have just read this interesting discussion thread at ProZ. The posters make a good point: all upper case letters can appear offensive to the reader. I, for one say that it is a good idea to mirror the source document when translating legal texts. I feel that it makes them look more "authentic" albeit "annoying" to some.
The Yndigo blog has a post on typography that talks about where to draw the line when it comes to copying the original formatting of the source document. (The all caps debate would fall into this category).