One woman who has been keeping journals for many years, began some time ago going through them and creating summaries, abstracting events, to make it easier, down the road, for her family to access and read her journals. While I firmly believe her family will want to read the full text of her journals, witness her world, and see the world through her eyes, the summary would allow them to gain an essence of their content more quickly, and read through them selectively. She is also creating summaries of other experiences she has written about and materials she has inherited from her own ancestors, again to create a more ready access.
Summarizing is especially good when you have a lot of disparate material to present. Journals are, for the most part, written chronologically (even if there's a time lapse between entries) and they have context. Military records and other types of documents might not be as easy to read, so it makes sense to extract pertinent information and incorporate that into a life story or a chronology of someone's life. This is what professional genealogists do in their reports. They summarize the main features of the material researched, carefully document where the information derived, and create a cohesive, readable document. But the originals are still there for reference. Summarizing journals are something else.
While I have not created summaries my own journals, I have created a table of contents that serves a similar purpose, indicating the dates covered within a particular journal and the main events during that time period -- a wedding, the birth of a child, graduation, etc. And I actually did that for my own use; I sometimes had to search at length for something I knew to be in one of my journals, so the table of contents helps me refer back to these events. Summarizing takes it one step further. The only problem in condensing, is losing the thoughts and feelings that tend to emerge as we write, which is, after all, the essence journal writing. In summarizing, we might tend to highlight the upbeat and positive, glossing over life's challenges and how we managed.
But so long as the summary does not "replace" the full text -- in other words, we keep the original, we can be fairly certain those who follow will want to read it all and discover, line upon line -- and between the lines -- the experiences and lessons of a life lived.