Hmm, perhaps I assumed too much.
My writers knew me VERY well. They knew my academic history, where I was currently working/had been working, had hard copies of my resume, had a list of the schools I was applying to (obviously) and a list that clearly stated each school's LOR due date. I think it's fair to assume that the people who are writing on your behalf know what you're doing and what you're about. Why else would you select them? And obviously, if they ask for more in-depth information and some guidance with the letter writing process, give it to them. Now that I think back, I did have a brief chat with one of my writers about what my end-goal was so he could tailor my letter a little more, but given that I worked for the man, he basically knew what was going on before our conversation. I also applied to all generalist programs (certificates were selected after the fact). Perhaps I'm an exception to the rule and have been lucky. The folks I selected are intelligent people, so I trusted their abilities to write a letter that they were already extremely familiar with. I was in a small graduate program, so perhaps it was a more intimate setting? Who knows. Is having faith in their abilites the worst advice ever? Doubtful. Especially considering it's worked out for me quite well a few times now.
Coming out of undergrad, or being out awhile, I suppose things could be different. Give them whatever info you think is pertinent, but just know you won't be given the prompt and therefore you can't totally coach them.